[PATCH] D94268: Allow _mm_empty() (via llvm.x86.mmx.emms) to be a no-op without MMX.

[James Y Knight via Phabricator via cfe-commits]

jyknight created this revision.
jyknight added reviewers: craig.topper, spatel, RKSimon.
Herald added subscribers: pengfei, hiraditya.
jyknight requested review of this revision.
Herald added projects: clang, LLVM.
Herald added subscribers: llvm-commits, cfe-commits.

In Clang, the other "MMX" intrinsic functions are being migrated to
SSE2, and will thus be usable even when compiling with -mno-mmx. These
SSE2 implementations don't require the use of _mm_empty(), but
existing (properly-written) code will still have calls to
_mm_empty(). It's therefore desirable to make the function a no-op in
this mode.

The function cannot be made a no-op universally, however, because MMX
may still be used by inline assembly. Therefore, have _mm_empty be
usable both with and without MMX -- and emit the LLVM intrinsic in
both cases, but cause the llvm intrinsic to generate an EMMS
instruction only if MMX is actually enabled.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D94268

Files:
  clang/include/clang/Basic/BuiltinsX86.def
  clang/lib/Headers/mmintrin.h
  llvm/lib/Target/X86/X86ISelLowering.cpp
  llvm/test/CodeGen/X86/mmx-emms.ll


Index: llvm/test/CodeGen/X86/mmx-emms.ll
===================================================================
--- /dev/null
+++ llvm/test/CodeGen/X86/mmx-emms.ll
@@ -0,0 +1,21 @@
+; RUN: llc -mcpu=i686 -mattr=+mmx < %s | FileCheck %s --check-prefixes=CHECK,CHECK-MMX
+; RUN: llc -mcpu=i686 -mattr=-mmx < %s | FileCheck %s --check-prefixes=CHECK
+; RUN: llc -mcpu=i686 -mattr=+sse2 < %s | FileCheck %s --check-prefixes=CHECK
+
+target datalayout = "e-p:32:32:32-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:32:64-f32:32:32"
+target triple = "i386-pc-linux-gnu"
+
+;; Verify that the llvm.x86.mmx.emms intrinsic works whether or not
+;; MMX is enabled, but that it doesn't emit any instructions if mmx is
+;; disabled.
+
+;; CHECK-LABEL: mmx_emms:
+;; CHECK: # %bb.0:
+;; CHECK-MMX-NEXT: emms
+;; CHECK-NEXT: retl
+define void @mmx_emms() {
+  tail call void @llvm.x86.mmx.emms() nounwind
+  ret void
+}
+
+declare void @llvm.x86.mmx.emms() nounwind
Index: llvm/lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- llvm/lib/Target/X86/X86ISelLowering.cpp
+++ llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -26170,6 +26170,14 @@
       return DAG.getNode(ISD::MERGE_VALUES, dl, Op->getVTList(), SetCC,
                          Operation.getValue(1));
     }
+    case Intrinsic::x86_mmx_emms: {
+      // Emit nothing for the EMMS intrinsic when MMX is disabled.
+      if (!Subtarget.hasMMX()) {
+        SDValue Chain = Op.getOperand(0);
+        return Chain;
+      }
+      return SDValue();
+    }
     }
     return SDValue();
   }
Index: clang/lib/Headers/mmintrin.h
===================================================================
--- clang/lib/Headers/mmintrin.h
+++ clang/lib/Headers/mmintrin.h
@@ -41,14 +41,15 @@
 #define __anyext128(x) (__m128i)__builtin_shufflevector((__v2si)(x), __extension__ (__v2si){}, 0, 1, -1, -1)
 #define __extract2_32(a) (__m64)__builtin_shufflevector((__v4si)(a), __extension__ (__v4si){}, 0, 2);
 
-/// Clears the MMX state by setting the state of the x87 stack registers
-///    to empty.
+/// Clears the MMX state by setting the state of the x87 stack registers to
+///    empty. This intrinsic is accepted but emits no instructions if MMX is
+///    disabled at compile-time (e.g. via -mno-mmx).
 ///
 /// \headerfile <x86intrin.h>
 ///
 /// This intrinsic corresponds to the <c> EMMS </c> instruction.
 ///
-static __inline__ void  __attribute__((__always_inline__, __nodebug__, __target__("mmx")))
+static __inline__ void  __attribute__((__always_inline__, __nodebug__))
 _mm_empty(void)
 {
     __builtin_ia32_emms();
Index: clang/include/clang/Basic/BuiltinsX86.def
===================================================================
--- clang/include/clang/Basic/BuiltinsX86.def
+++ clang/include/clang/Basic/BuiltinsX86.def
@@ -83,7 +83,7 @@
 // MMX usage is no longer supported in Clang; all of the formerly "MMX"
 // intrinsic functions are now expanded into SSE2 code in the headers.
 
-TARGET_BUILTIN(__builtin_ia32_emms, "v", "n", "mmx")
+TARGET_BUILTIN(__builtin_ia32_emms, "v", "n", "")
 TARGET_BUILTIN(__builtin_ia32_vec_ext_v4hi, "sV4sIi", "ncV:64:", "sse")
 TARGET_BUILTIN(__builtin_ia32_vec_set_v4hi, "V4sV4ssIi", "ncV:64:", "sse")
 


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