From 87a6dff094a32ff477404bfea42d9e8b02ad8176 Mon Sep 17 00:00:00 2001
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Wed, 20 Feb 2019 18:46:34 -0800
Subject: [PATCH] cpu_has_eflag(): eliminate if the answer is already known

There is no need to test for a flag if the answer can be given at
compile time.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
---
 com32/include/x86/cpu.h  |  8 +++++++-
 com32/include/x86/regs.h | 13 +++++++++++++
 2 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/com32/include/x86/cpu.h b/com32/include/x86/cpu.h
index 5bdc74de..5537da66 100644
--- a/com32/include/x86/cpu.h
+++ b/com32/include/x86/cpu.h
@@ -4,11 +4,17 @@
 #include <klibc/compiler.h>
 #include <inttypes.h>
 #include <stdbool.h>
+#include <x86/regs.h>
 
-static inline bool cpu_has_eflag(unsigned long flag)
+static inline __constfunc bool cpu_has_eflag(unsigned long flag)
 {
     unsigned long f1, f2;
 
+    if (__builtin_constant_p(flag)) {
+	if (!(flag & ~(unsigned long)KNOWN_EFLAGS))
+	    return true;
+    }
+
     asm("pushf ; "
 	"pushf ; "
 	"pop %0 ; "
diff --git a/com32/include/x86/regs.h b/com32/include/x86/regs.h
index 62f3a53d..d774c724 100644
--- a/com32/include/x86/regs.h
+++ b/com32/include/x86/regs.h
@@ -24,6 +24,19 @@
 #define EFLAGS_VIP	0x00100000
 #define EFLAGS_ID	0x00200000
 
+/* Flags known to be present */
+#define I386_KNOWN_EFLAGS \
+    (EFLAGS_CF|EFLAGS_PF|EFLAGS_AF|EFLAGS_ZF|EFLAGS_SF|EFLAGS_TF|\
+     EFLAGS_IF|EFLAGS_DF|EFLAGS_OF|EFLAGS_IOPL|EFLAGS_NT|EFLAGS_RF)
+#define X86_64_KNOWN_EFLAGS \
+    (I386_KNOWN_EFLAGS|EFLAGS_AC|EFLAGS_ID)
+
+#ifdef __x86_64__
+# define KNOWN_EFLAGS X86_64_KNOWN_EFLAGS
+#else
+# define KNOWN_EFLAGS I386_KNOWN_EFLAGS
+#endif
+
 /* CR0 definitions */
 #define CR0_PE		0x00000001
 #define CR0_MP  	0x00000002
-- 
2.11.4.GIT