[PATCH v4 2/3] Do not mistreat instructions as cmpxchg8b

[Pierre Marsais <pierre.marsais@lse.epita.fr>]

All x86 instructions starting with opcode 0x0f7c where considered as
cmpxchg8b if ir.mod == 3, regardless of ir.reg. However, there are some
instructions (such as xsavec) sharing the same opcode, but with
different ir.reg.

This change throws an error when recording on unsupported instructions
instead of considering them as cmpxchg8b.

gdb/ChangeLog:

2018-10-05  Pierre Marsais <pierre.marsais@lse.epita.fr>

	* i386-tdep.c: (i386_process_record): Improve decoding of
	instructions starting with 0x0f7c.
---
 gdb/i386-tdep.c | 76 ++++++++++++++++++++++++++++---------------------
 1 file changed, 43 insertions(+), 33 deletions(-)

diff --git a/gdb/i386-tdep.c b/gdb/i386-tdep.c
index a9fe290307..90c78e0bbc 100644
--- a/gdb/i386-tdep.c
+++ b/gdb/i386-tdep.c
@@ -5477,39 +5477,49 @@ i386_process_record (struct gdbarch *gdbarch, struct regcache *regcache,
     case 0x0fc7:    /* cmpxchg8b / rdrand / rdseed */
       if (i386_record_modrm (&ir))
 	return -1;
-      if (ir.mod == 3)
-	{
-	  /* rdrand and rdseed use the 3 bits of the REG field of ModR/M as
-	     an extended opcode.  rdrand has bits 110 (/6) and rdseed
-	     has bits 111 (/7).  */
-	  if (ir.reg == 6 || ir.reg == 7)
-	    {
-	      /* The storage register is described by the 3 R/M bits, but the
-		 REX.B prefix may be used to give access to registers
-		 R8~R15.  In this case ir.rex_b + R/M will give us the register
-		 in the range R8~R15.
-
-		 REX.W may also be used to access 64-bit registers, but we
-		 already record entire registers and not just partial bits
-		 of them.  */
-	      I386_RECORD_FULL_ARCH_LIST_ADD_REG (ir.rex_b + ir.rm);
-	      /* These instructions also set conditional bits.  */
-	      I386_RECORD_FULL_ARCH_LIST_ADD_REG (X86_RECORD_EFLAGS_REGNUM);
-	      break;
-	    }
-	  else
-	    {
-	      /* We don't handle this particular instruction yet.  */
-	      ir.addr -= 2;
-	      opcode = opcode << 8 | ir.modrm;
-	      goto no_support;
-	    }
-	}
-      I386_RECORD_FULL_ARCH_LIST_ADD_REG (X86_RECORD_REAX_REGNUM);
-      I386_RECORD_FULL_ARCH_LIST_ADD_REG (X86_RECORD_REDX_REGNUM);
-      if (i386_record_lea_modrm (&ir))
-	return -1;
-      I386_RECORD_FULL_ARCH_LIST_ADD_REG (X86_RECORD_EFLAGS_REGNUM);
+      switch (ir.reg) {
+        case 1: /* cmpxchg8b */
+          if (ir.mod == 3)
+            {
+              ir.addr -= 2;
+              opcode = opcode << 8 | ir.modrm;
+              goto no_support;
+            }
+          I386_RECORD_FULL_ARCH_LIST_ADD_REG (X86_RECORD_REAX_REGNUM);
+          I386_RECORD_FULL_ARCH_LIST_ADD_REG (X86_RECORD_REDX_REGNUM);
+          if (i386_record_lea_modrm (&ir))
+            return -1;
+          I386_RECORD_FULL_ARCH_LIST_ADD_REG (X86_RECORD_EFLAGS_REGNUM);
+          break;
+        case 6: /* rdrand */
+        case 7: /* rdseed */
+          if (ir.mod != 3)
+            {
+              /* We don't handle this particular instruction yet.  */
+              ir.addr -= 2;
+              opcode = opcode << 8 | ir.modrm;
+              goto no_support;
+            }
+          /* rdrand and rdseed use the 3 bits of the REG field of ModR/M as
+             an extended opcode.  rdrand has bits 110 (/6) and rdseed
+             has bits 111 (/7).  */
+          /* The storage register is described by the 3 R/M bits, but the
+             REX.B prefix may be used to give access to registers
+             R8~R15.  In this case ir.rex_b + R/M will give us the register
+             in the range R8~R15.
+
+             REX.W may also be used to access 64-bit registers, but we
+             already record entire registers and not just partial bits
+             of them.  */
+          I386_RECORD_FULL_ARCH_LIST_ADD_REG (ir.rex_b + ir.rm);
+          /* These instructions also set conditional bits.  */
+          I386_RECORD_FULL_ARCH_LIST_ADD_REG (X86_RECORD_EFLAGS_REGNUM);
+          break;
+        default:
+          ir.addr -= 2;
+          opcode = opcode << 8 | ir.modrm;
+          goto no_support;
+      }
       break;
 
     case 0x50:    /* push */
-- 
2.19.0