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[mono-project.git] / mono / mini / exceptions-ppc.c

/*
 * exceptions-ppc.c: exception support for PowerPC
 *
 * Authors:
 *   Dietmar Maurer (dietmar@ximian.com)
 *   Paolo Molaro (lupus@ximian.com)
 *   Andreas Faerber <andreas.faerber@web.de>
 *
 * (C) 2001 Ximian, Inc.
 * (C) 2007-2008 Andreas Faerber
 */

#include <config.h>
#include <glib.h>
#include <signal.h>
#include <string.h>
#include <stddef.h>
#if HAVE_UCONTEXT_H
#include <ucontext.h>
#endif

#include <mono/arch/ppc/ppc-codegen.h>
#include <mono/metadata/appdomain.h>
#include <mono/metadata/tabledefs.h>
#include <mono/metadata/threads.h>
#include <mono/metadata/debug-helpers.h>
#include <mono/metadata/exception.h>
#include <mono/metadata/mono-debug.h>

#include "mini.h"
#include "mini-ppc.h"

/*

struct sigcontext {
    int      sc_onstack;     // sigstack state to restore 
    int      sc_mask;        // signal mask to restore 
    int      sc_ir;          // pc 
    int      sc_psw;         // processor status word 
    int      sc_sp;          // stack pointer if sc_regs == NULL 
    void    *sc_regs;        // (kernel private) saved state 
};

struct ucontext {
        int             uc_onstack;
        sigset_t        uc_sigmask;     // signal mask used by this context 
        stack_t         uc_stack;       // stack used by this context 
        struct ucontext *uc_link;       // pointer to resuming context 
        size_t          uc_mcsize;      // size of the machine context passed in 
        mcontext_t      uc_mcontext;    // machine specific context 
};

typedef struct ppc_exception_state {
        unsigned long dar;      // Fault registers for coredump 
        unsigned long dsisr;
        unsigned long exception;// number of powerpc exception taken 
        unsigned long pad0;     // align to 16 bytes 

        unsigned long pad1[4];  // space in PCB "just in case" 
} ppc_exception_state_t;

typedef struct ppc_vector_state {
        unsigned long   save_vr[32][4];
        unsigned long   save_vscr[4];
        unsigned int    save_pad5[4];
        unsigned int    save_vrvalid;                   // VRs that have been saved 
        unsigned int    save_pad6[7];
} ppc_vector_state_t;

typedef struct ppc_float_state {
        double  fpregs[32];

        unsigned int fpscr_pad; // fpscr is 64 bits, 32 bits of rubbish 
        unsigned int fpscr;     // floating point status register 
} ppc_float_state_t;

typedef struct ppc_thread_state {
        unsigned int srr0;      // Instruction address register (PC) 
        unsigned int srr1;      // Machine state register (supervisor) 
        unsigned int r0;
        unsigned int r1;
        unsigned int r2;
        ... 
        unsigned int r31;
        unsigned int cr;        // Condition register 
        unsigned int xer;       // User's integer exception register 
        unsigned int lr;        // Link register 
        unsigned int ctr;       // Count register 
        unsigned int mq;        // MQ register (601 only) 

        unsigned int vrsave;    // Vector Save Register 
} ppc_thread_state_t;

struct mcontext {
        ppc_exception_state_t   es;
        ppc_thread_state_t      ss;
        ppc_float_state_t       fs;
        ppc_vector_state_t      vs;
};

typedef struct mcontext  * mcontext_t;

Linux/PPC instead has:
struct sigcontext {
        unsigned long   _unused[4];
        int             signal;
        unsigned long   handler;
        unsigned long   oldmask;
        struct pt_regs  *regs;
};
struct pt_regs {
        unsigned long gpr[32];
        unsigned long nip;
        unsigned long msr;
        unsigned long orig_gpr3;        // Used for restarting system calls 
        unsigned long ctr;
        unsigned long link;
        unsigned long xer;
        unsigned long ccr;
        unsigned long mq;               // 601 only (not used at present) 
                                        // Used on APUS to hold IPL value. 
        unsigned long trap;             // Reason for being here 
        // N.B. for critical exceptions on 4xx, the dar and dsisr
        // fields are overloaded to hold srr0 and srr1. 
        unsigned long dar;              // Fault registers 
        unsigned long dsisr;            // on 4xx/Book-E used for ESR 
        unsigned long result;           // Result of a system call 
};
struct mcontext {
        elf_gregset_t   mc_gregs;
        elf_fpregset_t  mc_fregs;
        unsigned long   mc_pad[2];
        elf_vrregset_t  mc_vregs __attribute__((__aligned__(16)));
};

struct ucontext {
        unsigned long    uc_flags;
        struct ucontext *uc_link;
        stack_t          uc_stack;
        int              uc_pad[7];
        struct mcontext *uc_regs;       // points to uc_mcontext field 
        sigset_t         uc_sigmask;
        // glibc has 1024-bit signal masks, ours are 64-bit 
        int              uc_maskext[30];
        int              uc_pad2[3];
        struct mcontext  uc_mcontext;
};

#define ELF_NGREG       48      // includes nip, msr, lr, etc. 
#define ELF_NFPREG      33      // includes fpscr 

// General registers 
typedef unsigned long elf_greg_t;
typedef elf_greg_t elf_gregset_t[ELF_NGREG];

// Floating point registers 
typedef double elf_fpreg_t;
typedef elf_fpreg_t elf_fpregset_t[ELF_NFPREG];


*/


#define restore_regs_from_context(ctx_reg,ip_reg,tmp_reg) do {  \
                int reg;        \
                ppc_load_reg (code, ip_reg, G_STRUCT_OFFSET (MonoContext, sc_ir), ctx_reg);     \
                ppc_load_multiple_regs (code, ppc_r13, G_STRUCT_OFFSET (MonoContext, regs), ctx_reg);   \
                for (reg = 0; reg < MONO_SAVED_FREGS; ++reg) {  \
                        ppc_lfd (code, (14 + reg),      \
                                G_STRUCT_OFFSET(MonoContext, fregs) + reg * sizeof (gdouble), ctx_reg); \
                }       \
        } while (0)

/* nothing to do */
#define setup_context(ctx)

#ifdef PPC_USES_FUNCTION_DESCRIPTOR
guint8*
mono_ppc_create_pre_code_ftnptr (guint8 *code)
{
        MonoPPCFunctionDescriptor *ftnptr = (MonoPPCFunctionDescriptor*)code;

        code += sizeof (MonoPPCFunctionDescriptor);
        ftnptr->code = code;
        ftnptr->toc = NULL;
        ftnptr->env = NULL;

        return code;
}
#endif

/*
 * arch_get_restore_context:
 *
 * Returns a pointer to a method which restores a previously saved sigcontext.
 * The first argument in r3 is the pointer to the context.
 */
gpointer
mono_arch_get_restore_context (void)
{
        guint8 *start, *code;
        int size = MONO_PPC_32_64_CASE (128, 172) + PPC_FTNPTR_SIZE;

        code = start = mono_global_codeman_reserve (size);
        code = mono_ppc_create_pre_code_ftnptr (code);
        restore_regs_from_context (ppc_r3, ppc_r4, ppc_r5);
        /* restore also the stack pointer */
        ppc_load_reg (code, ppc_sp, G_STRUCT_OFFSET (MonoContext, sc_sp), ppc_r3);
        //ppc_break (code);
        /* jump to the saved IP */
        ppc_mtctr (code, ppc_r4);
        ppc_bcctr (code, PPC_BR_ALWAYS, 0);
        /* never reached */
        ppc_break (code);

        g_assert ((code - start) <= size);
        mono_arch_flush_icache (start, code - start);
        return start;
}

#define SAVED_REGS_LENGTH               (sizeof (gdouble) * MONO_SAVED_FREGS + sizeof (gpointer) * MONO_SAVED_GREGS)
#define ALIGN_STACK_FRAME_SIZE(s)       (((s) + MONO_ARCH_FRAME_ALIGNMENT - 1) & ~(MONO_ARCH_FRAME_ALIGNMENT - 1))
/* The 64 bytes here are for outgoing arguments and a bit of spare.
   We don't use it all, but it doesn't hurt. */
#define REG_SAVE_STACK_FRAME_SIZE       (ALIGN_STACK_FRAME_SIZE (SAVED_REGS_LENGTH + PPC_MINIMAL_STACK_SIZE + 64))

static guint8*
emit_save_saved_regs (guint8 *code, int pos)
{
        int i;

        for (i = 31; i >= 14; --i) {
                pos -= sizeof (gdouble);
                ppc_stfd (code, i, pos, ppc_sp);
        }
        pos -= sizeof (gpointer) * MONO_SAVED_GREGS;
        ppc_store_multiple_regs (code, ppc_r13, pos, ppc_sp);

        return code;
}

/*
 * mono_arch_get_call_filter:
 *
 * Returns a pointer to a method which calls an exception filter. We
 * also use this function to call finally handlers (we pass NULL as 
 * @exc object in this case).
 */
gpointer
mono_arch_get_call_filter (void)
{
        guint8 *start, *code;
        int alloc_size, pos, i;
        int size = MONO_PPC_32_64_CASE (320, 500) + PPC_FTNPTR_SIZE;

        /* call_filter (MonoContext *ctx, unsigned long eip, gpointer exc) */
        code = start = mono_global_codeman_reserve (size);
        code = mono_ppc_create_pre_code_ftnptr (code);

        /* store ret addr */
        ppc_mflr (code, ppc_r0);
        ppc_store_reg (code, ppc_r0, PPC_RET_ADDR_OFFSET, ppc_sp);

        alloc_size = REG_SAVE_STACK_FRAME_SIZE;

        /* allocate stack frame and set link from sp in ctx */
        g_assert ((alloc_size & (MONO_ARCH_FRAME_ALIGNMENT-1)) == 0);
        ppc_load_reg (code, ppc_r0, G_STRUCT_OFFSET (MonoContext, sc_sp), ppc_r3);
        ppc_load_reg_indexed (code, ppc_r0, ppc_r0, ppc_r0);
        ppc_store_reg_update (code, ppc_r0, -alloc_size, ppc_sp);

        code = emit_save_saved_regs (code, alloc_size);

        /* restore all the regs from ctx (in r3), but not r1, the stack pointer */
        restore_regs_from_context (ppc_r3, ppc_r6, ppc_r7);
        /* call handler at eip (r4) and set the first arg with the exception (r5) */
        ppc_mtctr (code, ppc_r4);
        ppc_mr (code, ppc_r3, ppc_r5);
        ppc_bcctrl (code, PPC_BR_ALWAYS, 0);

        /* epilog */
        ppc_load_reg (code, ppc_r0, alloc_size + PPC_RET_ADDR_OFFSET, ppc_sp);
        ppc_mtlr (code, ppc_r0);

        /* restore all the regs from the stack */
        pos = alloc_size;
        for (i = 31; i >= 14; --i) {
                pos -= sizeof (gdouble);
                ppc_lfd (code, i, pos, ppc_sp);
        }
        pos -= sizeof (gpointer) * MONO_SAVED_GREGS;
        ppc_load_multiple_regs (code, ppc_r13, pos, ppc_sp);

        ppc_addic (code, ppc_sp, ppc_sp, alloc_size);
        ppc_blr (code);

        g_assert ((code - start) < size);
        mono_arch_flush_icache (start, code - start);
        return start;
}

static void
throw_exception (MonoObject *exc, unsigned long eip, unsigned long esp, gulong *int_regs, gdouble *fp_regs, gboolean rethrow)
{
        static void (*restore_context) (MonoContext *);
        MonoContext ctx;

        if (!restore_context)
                restore_context = mono_get_restore_context ();

        /* adjust eip so that it point into the call instruction */
        eip -= 4;

        setup_context (&ctx);

        /*printf ("stack in throw: %p\n", esp);*/
        MONO_CONTEXT_SET_BP (&ctx, esp);
        MONO_CONTEXT_SET_IP (&ctx, eip);
        memcpy (&ctx.regs, int_regs, sizeof (gulong) * MONO_SAVED_GREGS);
        memcpy (&ctx.fregs, fp_regs, sizeof (double) * MONO_SAVED_FREGS);

        if (mono_object_isinst (exc, mono_defaults.exception_class)) {
                MonoException *mono_ex = (MonoException*)exc;
                if (!rethrow)
                        mono_ex->stack_trace = NULL;
        }
        mono_handle_exception (&ctx, exc, (gpointer)eip, FALSE);
        restore_context (&ctx);

        g_assert_not_reached ();
}

/**
 * arch_get_throw_exception_generic:
 *
 * Returns a function pointer which can be used to raise 
 * exceptions. The returned function has the following 
 * signature: void (*func) (MonoException *exc); or
 * void (*func) (guint32 ex_token, gpointer ip)
 *
 */
static gpointer 
mono_arch_get_throw_exception_generic (int size, int corlib, gboolean rethrow)
{
        guint8 *start, *code;
        int alloc_size, pos;

        start = mono_global_codeman_reserve (size);

        code = mono_ppc_create_pre_code_ftnptr (start);

        /* store ret addr */
        if (corlib)
                ppc_mr (code, ppc_r0, ppc_r4);
        else
                ppc_mflr (code, ppc_r0);
        ppc_store_reg (code, ppc_r0, PPC_RET_ADDR_OFFSET, ppc_sp);

        alloc_size = REG_SAVE_STACK_FRAME_SIZE;

        g_assert ((alloc_size & (MONO_ARCH_FRAME_ALIGNMENT-1)) == 0);
        ppc_store_reg_update (code, ppc_sp, -alloc_size, ppc_sp);

        code = emit_save_saved_regs (code, alloc_size);

        //ppc_break (code);
        if (corlib) {
                ppc_mr (code, ppc_r4, ppc_r3);
                ppc_load (code, ppc_r3, (gulong)mono_defaults.corlib);
                ppc_load_func (code, ppc_r0, mono_exception_from_token);
                ppc_mtctr (code, ppc_r0);
                ppc_bcctrl (code, PPC_BR_ALWAYS, 0);
        }

        /* call throw_exception (exc, ip, sp, int_regs, fp_regs) */
        /* caller sp */
        ppc_load_reg (code, ppc_r5, 0, ppc_sp);
        /* exc is already in place in r3 */
        if (corlib)
                ppc_load_reg (code, ppc_r4, PPC_RET_ADDR_OFFSET, ppc_r5);
        else
                ppc_mr (code, ppc_r4, ppc_r0); /* caller ip */
        /* pointer to the saved fp regs */
        pos = alloc_size - sizeof (gdouble) * MONO_SAVED_FREGS;
        ppc_addi (code, ppc_r7, ppc_sp, pos);
        /* pointer to the saved int regs */
        pos -= sizeof (gpointer) * MONO_SAVED_GREGS;
        ppc_addi (code, ppc_r6, ppc_sp, pos);
        ppc_li (code, ppc_r8, rethrow);

        ppc_load_func (code, ppc_r0, throw_exception);
        ppc_mtctr (code, ppc_r0);
        ppc_bcctrl (code, PPC_BR_ALWAYS, 0);
        /* we should never reach this breakpoint */
        ppc_break (code);
        g_assert ((code - start) <= size);
        mono_arch_flush_icache (start, code - start);
        return start;
}

/**
 * mono_arch_get_rethrow_exception:
 *
 * Returns a function pointer which can be used to rethrow 
 * exceptions. The returned function has the following 
 * signature: void (*func) (MonoException *exc); 
 *
 */
gpointer
mono_arch_get_rethrow_exception (void)
{
        int size = MONO_PPC_32_64_CASE (132, 224) + PPC_FTNPTR_SIZE;

        return mono_arch_get_throw_exception_generic (size, FALSE, TRUE);
}
/**
 * arch_get_throw_exception:
 *
 * Returns a function pointer which can be used to raise 
 * exceptions. The returned function has the following 
 * signature: void (*func) (MonoException *exc); 
 * For example to raise an arithmetic exception you can use:
 *
 * x86_push_imm (code, mono_get_exception_arithmetic ()); 
 * x86_call_code (code, arch_get_throw_exception ()); 
 *
 */
gpointer 
mono_arch_get_throw_exception (void)
{
        int size = MONO_PPC_32_64_CASE (132, 224) + PPC_FTNPTR_SIZE;

        return mono_arch_get_throw_exception_generic (size, FALSE, FALSE);
}

/**
 * arch_get_throw_exception_by_name:
 *
 * Returns a function pointer which can be used to raise 
 * corlib exceptions. The returned function has the following 
 * signature: void (*func) (char *exc_name); 
 * For example to raise an arithmetic exception you can use:
 *
 * x86_push_imm (code, "ArithmeticException"); 
 * x86_call_code (code, arch_get_throw_exception_by_name ()); 
 *
 */
gpointer 
mono_arch_get_throw_exception_by_name (void)
{
        guint8 *start, *code;
        int size = 64;

        /* Not used on PPC */   
        start = code = mono_global_codeman_reserve (size);
        ppc_break (code);
        mono_arch_flush_icache (start, code - start);
        return start;
}

/**
 * mono_arch_get_throw_corlib_exception:
 *
 * Returns a function pointer which can be used to raise 
 * corlib exceptions. The returned function has the following 
 * signature: void (*func) (guint32 ex_token, guint32 offset); 
 * On PPC, we pass the ip instead of the offset
 */
gpointer 
mono_arch_get_throw_corlib_exception (void)
{
        int size = MONO_PPC_32_64_CASE (168, 304) + PPC_FTNPTR_SIZE;

        return mono_arch_get_throw_exception_generic (size, TRUE, FALSE);
}

/* mono_arch_find_jit_info:
 *
 * This function is used to gather information from @ctx. It return the 
 * MonoJitInfo of the corresponding function, unwinds one stack frame and
 * stores the resulting context into @new_ctx. It also stores a string 
 * describing the stack location into @trace (if not NULL), and modifies
 * the @lmf if necessary. @native_offset return the IP offset from the 
 * start of the function or -1 if that info is not available.
 */
MonoJitInfo *
mono_arch_find_jit_info (MonoDomain *domain, MonoJitTlsData *jit_tls, MonoJitInfo *res, MonoJitInfo *prev_ji, MonoContext *ctx,
                                                 MonoContext *new_ctx, MonoLMF **lmf, gboolean *managed)
{
        MonoJitInfo *ji;
        gpointer ip = MONO_CONTEXT_GET_IP (ctx);
        MonoPPCStackFrame *sframe;

        /* Avoid costly table lookup during stack overflow */
        if (prev_ji && (ip > prev_ji->code_start && ((guint8*)ip < ((guint8*)prev_ji->code_start) + prev_ji->code_size)))
                ji = prev_ji;
        else
                ji = mono_jit_info_table_find (domain, ip);

        if (managed)
                *managed = FALSE;

        if (ji != NULL) {
                gint32 address;
                int offset, i;

                *new_ctx = *ctx;
                setup_context (new_ctx);

                if (*lmf && (MONO_CONTEXT_GET_SP (ctx) >= (gpointer)(*lmf)->ebp)) {
                        /* remove any unused lmf */
                        *lmf = (*lmf)->previous_lmf;
                }

                address = (char *)ip - (char *)ji->code_start;

                if (managed)
                        if (!ji->method->wrapper_type)
                                *managed = TRUE;

                sframe = (MonoPPCStackFrame*)MONO_CONTEXT_GET_SP (ctx);
                MONO_CONTEXT_SET_BP (new_ctx, sframe->sp);
                if (ji->method->save_lmf) {
                        memcpy (&new_ctx->fregs, (char*)sframe->sp - sizeof (double) * MONO_SAVED_FREGS, sizeof (double) * MONO_SAVED_FREGS);
                        memcpy (&new_ctx->regs, (char*)sframe->sp - sizeof (double) * MONO_SAVED_FREGS - sizeof (gulong) * MONO_SAVED_GREGS, sizeof (gulong) * MONO_SAVED_GREGS);
                } else if (ji->used_regs) {
                        /* keep updated with emit_prolog in mini-ppc.c */
                        offset = 0;
                        /* FIXME handle floating point args 
                        for (i = 31; i >= 14; --i) {
                                if (ji->used_fregs & (1 << i)) {
                                        offset += sizeof (double);
                                        new_ctx->fregs [i - 14] = *(gulong*)((char*)sframe->sp - offset);
                                }
                        }*/
                        for (i = 31; i >= 13; --i) {
                                if (ji->used_regs & (1 << i)) {
                                        offset += sizeof (gulong);
                                        new_ctx->regs [i - 13] = *(gulong*)((char*)sframe->sp - offset);
                                }
                        }
                }
                /* the calling IP is in the parent frame */
                sframe = (MonoPPCStackFrame*)sframe->sp;
                /* we substract 4, so that the IP points into the call instruction */
                MONO_CONTEXT_SET_IP (new_ctx, sframe->lr - 4);

                return ji;
        } else if (*lmf) {
                
                *new_ctx = *ctx;
                setup_context (new_ctx);

                if ((ji = mono_jit_info_table_find (domain, (gpointer)(*lmf)->eip))) {
                } else {
                        if (!(*lmf)->method)
                                return (gpointer)-1;

                        memset (res, 0, sizeof (MonoJitInfo));
                        res->method = (*lmf)->method;
                }

                /*sframe = (MonoPPCStackFrame*)MONO_CONTEXT_GET_SP (ctx);
                MONO_CONTEXT_SET_BP (new_ctx, sframe->sp);
                MONO_CONTEXT_SET_IP (new_ctx, sframe->lr);*/
                MONO_CONTEXT_SET_BP (new_ctx, (*lmf)->ebp);
                MONO_CONTEXT_SET_IP (new_ctx, (*lmf)->eip);
                memcpy (&new_ctx->regs, (*lmf)->iregs, sizeof (gulong) * MONO_SAVED_GREGS);
                memcpy (&new_ctx->fregs, (*lmf)->fregs, sizeof (double) * MONO_SAVED_FREGS);

                /* FIXME: what about trampoline LMF frames?  see exceptions-x86.c */

                *lmf = (*lmf)->previous_lmf;

                return ji ? ji : res;
        }

        return NULL;
}

/*
 * This is the function called from the signal handler
 */
void
mono_arch_sigctx_to_monoctx (void *ctx, MonoContext *mctx)
{
        os_ucontext *uc = ctx;

        mctx->sc_ir = UCONTEXT_REG_NIP(uc);
        mctx->sc_sp = UCONTEXT_REG_Rn(uc, 1);
        memcpy (&mctx->regs, &UCONTEXT_REG_Rn(uc, 13), sizeof (gulong) * MONO_SAVED_GREGS);
        memcpy (&mctx->fregs, &UCONTEXT_REG_FPRn(uc, 14), sizeof (double) * MONO_SAVED_FREGS);
}

void
mono_arch_monoctx_to_sigctx (MonoContext *mctx, void *ctx)
{
        os_ucontext *uc = ctx;

        UCONTEXT_REG_NIP(uc) = mctx->sc_ir;
        UCONTEXT_REG_Rn(uc, 1) = mctx->sc_sp;
        memcpy (&UCONTEXT_REG_Rn(uc, 13), &mctx->regs, sizeof (gulong) * MONO_SAVED_GREGS);
        memcpy (&UCONTEXT_REG_FPRn(uc, 14), &mctx->fregs, sizeof (double) * MONO_SAVED_FREGS);
}

gpointer
mono_arch_ip_from_context (void *sigctx)
{
        os_ucontext *uc = sigctx;
        return (gpointer)UCONTEXT_REG_NIP(uc);
}

static void
altstack_handle_and_restore (void *sigctx, gpointer obj, gboolean test_only)
{
        void (*restore_context) (MonoContext *);
        MonoContext mctx;

        restore_context = mono_get_restore_context ();
        mono_arch_sigctx_to_monoctx (sigctx, &mctx);
        mono_handle_exception (&mctx, obj, (gpointer)mctx.sc_ir, test_only);
        restore_context (&mctx);
}

void
mono_arch_handle_altstack_exception (void *sigctx, gpointer fault_addr, gboolean stack_ovf)
{
#ifdef MONO_ARCH_USE_SIGACTION
        os_ucontext *uc = (ucontext_t*)sigctx;
        os_ucontext *uc_copy;
        MonoJitInfo *ji = mono_jit_info_table_find (mono_domain_get (), mono_arch_ip_from_context (sigctx));
        gpointer *sp;
        int frame_size;

        if (stack_ovf) {
                const char *method;
                /* we don't do much now, but we can warn the user with a useful message */
                fprintf (stderr, "Stack overflow: IP: %p, SP: %p\n", mono_arch_ip_from_context (sigctx), (gpointer)UCONTEXT_REG_Rn(uc, 1));
                if (ji && ji->method)
                        method = mono_method_full_name (ji->method, TRUE);
                else
                        method = "Unmanaged";
                fprintf (stderr, "At %s\n", method);
                abort ();
        }
        if (!ji)
                mono_handle_native_sigsegv (SIGSEGV, sigctx);
        /* setup a call frame on the real stack so that control is returned there
         * and exception handling can continue.
         * The frame looks like:
         *   ucontext struct
         *   ...
         * 224 is the size of the red zone
         */
        frame_size = sizeof (ucontext_t) + sizeof (gpointer) * 16 + 224;
        frame_size += 15;
        frame_size &= ~15;
        sp = (gpointer)(UCONTEXT_REG_Rn(uc, 1) & ~15);
        sp = (gpointer)((char*)sp - frame_size);
        /* may need to adjust pointers in the new struct copy, depending on the OS */
        uc_copy = (ucontext_t*)(sp + 16);
        memcpy (uc_copy, uc, sizeof (os_ucontext));
#if defined(__linux__) && !defined(__mono_ppc64__)
        uc_copy->uc_mcontext.uc_regs = (gpointer)((char*)uc_copy + ((char*)uc->uc_mcontext.uc_regs - (char*)uc));
#endif
        g_assert (mono_arch_ip_from_context (uc) == mono_arch_ip_from_context (uc_copy));
        /* at the return form the signal handler execution starts in altstack_handle_and_restore() */
        UCONTEXT_REG_LNK(uc) = UCONTEXT_REG_NIP(uc);
#ifdef PPC_USES_FUNCTION_DESCRIPTOR
        {
                MonoPPCFunctionDescriptor *handler_ftnptr = (MonoPPCFunctionDescriptor*)altstack_handle_and_restore;

                UCONTEXT_REG_NIP(uc) = (gulong)handler_ftnptr->code;
                UCONTEXT_REG_Rn(uc, 2) = (gulong)handler_ftnptr->toc;
        }
#else
        UCONTEXT_REG_NIP(uc) = (unsigned long)altstack_handle_and_restore;
#endif
        UCONTEXT_REG_Rn(uc, 1) = (unsigned long)sp;
        UCONTEXT_REG_Rn(uc, PPC_FIRST_ARG_REG) = (unsigned long)(sp + 16);
        UCONTEXT_REG_Rn(uc, PPC_FIRST_ARG_REG + 1) = 0;
        UCONTEXT_REG_Rn(uc, PPC_FIRST_ARG_REG + 2) = 0;
#endif
}

gboolean
mono_arch_handle_exception (void *ctx, gpointer obj, gboolean test_only)
{
        MonoContext mctx;
        gboolean result;

        mono_arch_sigctx_to_monoctx (ctx, &mctx);

        result = mono_handle_exception (&mctx, obj, (gpointer)mctx.sc_ir, test_only);
        /* restore the context so that returning from the signal handler will invoke
         * the catch clause 
         */
        mono_arch_monoctx_to_sigctx (&mctx, ctx);
        return result;
}

gboolean
mono_arch_has_unwind_info (gconstpointer addr)
{
        return FALSE;
}