[X86] Enable isel to use the PAUSE instruction even when SSE2 is disabled

Summary:
On older processors this instruction encoding is treated as a NOP.

MSVC doesn't disable intrinsics based on features the way clang/gcc does. Because the PAUSE instruction encoding doesn't crash older processors, some software out there uses these intrinsics without checking for SSE2.

This change also seems to also be consistent with gcc behavior.

Fixes PR34079

Reviewers: RKSimon, zvi

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D36361

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310190 91177308-0d34-0410-b5e6-96231b3b80d8

Author: topperc

lib/Target/X86/X86InstrSSE.td

@@ -3695,8 +3695,7 @@ let SchedRW = [WriteNop] in {
 // Pause. This "instruction" is encoded as "rep; nop", so even though it
 // was introduced with SSE2, it's backward compatible.
 def PAUSE : I<0x90, RawFrm, (outs), (ins),
-              "pause", [(int_x86_sse2_pause)], IIC_SSE_PAUSE>,
-              OBXS, Requires<[HasSSE2]>;
+              "pause", [(int_x86_sse2_pause)], IIC_SSE_PAUSE>, OBXS;
 }
 
 let SchedRW = [WriteFence] in {

test/CodeGen/X86/pause.ll

@@ -0,0 +1,15 @@
+; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
+; RUN: llc < %s -disable-peephole -mtriple=i386-apple-darwin -mattr=-sse -show-mc-encoding | FileCheck %s
+; RUN: llc < %s -disable-peephole -mtriple=i386-apple-darwin -mattr=-avx,+sse2 -show-mc-encoding | FileCheck %s
+; RUN: llc < %s -disable-peephole -mtriple=i386-apple-darwin -mattr=+avx2 -show-mc-encoding | FileCheck %s
+; RUN: llc < %s -disable-peephole -mtriple=i386-apple-darwin -mcpu=skx -show-mc-encoding | FileCheck %s
+
+define void @test_x86_sse2_pause() {
+; CHECK-LABEL: test_x86_sse2_pause:
+; CHECK:       ## BB#0:
+; CHECK-NEXT:    pause ## encoding: [0xf3,0x90]
+; CHECK-NEXT:    retl ## encoding: [0xc3]
+  tail call void @llvm.x86.sse2.pause()
+  ret void
+}
+declare void @llvm.x86.sse2.pause() nounwind