Define the x86 pause instruction.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@104204 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/X86/X86InstrSSE.td

@@ -2417,6 +2417,10 @@ def LFENCE : I<0xAE, MRM_E8, (outs), (ins),
 def MFENCE : I<0xAE, MRM_F0, (outs), (ins),
                "mfence", [(int_x86_sse2_mfence)]>, TB, Requires<[HasSSE2]>;
 
+// Pause. This "instruction" is encoded as "rep; nop", so even though it
+// was introduced with SSE2, it's backward compabitle.
+def PAUSE : I<0x90, RawFrm, (outs), (ins), "pause", []>, REP;
+
 //TODO: custom lower this so as to never even generate the noop
 def : Pat<(membarrier (i8 imm), (i8 imm), (i8 imm), (i8 imm),
            (i8 0)), (NOOP)>;

test/MC/AsmParser/X86/x86_32-bit_cat.s

@@ -2482,6 +2482,12 @@
 // CHECK: 	clflush	305419896
         	clflush	0x12345678
 
+// CHECK: 	pause
+        	pause
+
+// CHECK: 	sfence
+        	sfence
+
 // CHECK: 	lfence
         	lfence
 

test/MC/AsmParser/X86/x86_32-new-encoder.s

@@ -1,5 +1,8 @@
 // RUN: llvm-mc -triple i386-unknown-unknown --show-encoding %s | FileCheck %s
 
+	pause
+// CHECK: pause
+// CHECK: encoding: [0xf3,0x90]
 	sfence
 // CHECK: sfence
 // CHECK: encoding: [0x0f,0xae,0xf8]