From cf581cdca27885a72a57f8580a4e8093ed84c129 Mon Sep 17 00:00:00 2001
From: Douglas Katzman <dougk@google.com>
Date: Wed, 9 Aug 2017 14:17:53 -0400
Subject: [PATCH] x86-64: Decode FS: instruction prefix

Makes it possible to disassemble foreign code using __thread storage
without randomly losing sync.
---
 src/compiler/x86-64/insts.lisp        |  3 +++
 src/compiler/x86-64/target-insts.lisp |  4 ++--
 tests/assembler.pure.lisp             | 11 +++++++++++
 3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/src/compiler/x86-64/insts.lisp b/src/compiler/x86-64/insts.lisp
index 5ce45b058..2aed4b712 100644
--- a/src/compiler/x86-64/insts.lisp
+++ b/src/compiler/x86-64/insts.lisp
@@ -1460,6 +1460,9 @@
      #!+sb-thread
      (emit-byte segment #xf0))))
 
+(define-instruction fs (segment)
+  (:printer byte ((op #x64)) nil))
+
 (define-instruction lock (segment)
   (:printer byte ((op #b11110000)) nil))
 
diff --git a/src/compiler/x86-64/target-insts.lisp b/src/compiler/x86-64/target-insts.lisp
index e9d7f5aec..06fb7d421 100644
--- a/src/compiler/x86-64/target-insts.lisp
+++ b/src/compiler/x86-64/target-insts.lisp
@@ -128,8 +128,8 @@
       (princ value stream)))
 
 ;;; Return either a MACHINE-EA or a register (a fixnum).
-;;; VALUE is a list of the mod and r/m fields of the instruction's ModRM byte.
-;;; Depending on VALUE, a SIB byte and/or displacement may be read.
+;;; MOD and R/M are the extracted bits from the instruction's ModRM byte.
+;;; Depending on MOD and R/M, a SIB byte and/or displacement may be read.
 ;;; The REX.B and REX.X from dstate are appropriately consumed.
 (defun prefilter-reg/mem (dstate mod r/m)
   (declare (type disassem-state dstate)
diff --git a/tests/assembler.pure.lisp b/tests/assembler.pure.lisp
index 758c9b63f..b33c59493 100644
--- a/tests/assembler.pure.lisp
+++ b/tests/assembler.pure.lisp
@@ -121,3 +121,14 @@
     #+x86-64
     (try `(add ,rcx-tn ,(memref :qword)) "48034D00         ADD RCX, [$fp]")
     ))
+
+(with-test (:name :disassemble-fs-prefix :skipped-on '(not (or :x86-64)))
+  (let ((bytes (coerce '(#x64 #xF0 #x44 #x08 #x04 #x25 #x00 #x04 #x10 #x20)
+                       '(array (unsigned-byte 8) 1)))
+        (s (make-string-output-stream)))
+    (sb-sys:with-pinned-objects (bytes)
+      (sb-disassem::disassemble-memory (sb-sys:sap-int (sb-sys:vector-sap bytes))
+                                       (length bytes)
+                                       :stream s))
+    (assert (search "FS LOCK OR [#x20100400], R8B"
+                    (get-output-stream-string s)))))
-- 
2.11.4.GIT