[DAGCombiner] Teach visitMLOAD to replace an all ones mask with an unmasked load

[craig.topper]

Summary

If we have an all ones mask, we can just a regular masked load. InstCombine already gets this in IR. But the all ones mask can appear after type legalization.

Only avx512 test cases are affected because X86 backend already looks for element 0 and the last element being 1. It replaces this with an unmasked load and blend. The all ones mask is a special case of that where the blend will be removed. That transform is only enabled on avx2 targets. I believe that's because a non-zero passthru on avx2 already requires a separate blend so its more profitable to handle mixed constant masks.

This patch adds a dedicated all ones handling to the target independent DAG combiner. I've skipped extending, expanding, and index loads for now. X86 doesn't use index so I don't know much about it. Extending made me nervous because I wasn't sure I could trust the memory VT had the right element count due to some weirdness in vector splitting. For expanding I wasn't sure if we needed different undef handling.

[spatel]

LGTM

llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp

SDValue DAGCombiner::visitMLOAD(SDNode *N) {
   MaskedLoadSDNode *MLD = cast<MaskedLoadSDNode>(N);
   SDValue Mask = MLD->getMask();
   SDLoc DL(N);
 
   // Zap masked loads with a zero mask.
   if (ISD::isBuildVectorAllZeros(Mask.getNode()))
     return CombineTo(N, MLD->getPassThru(), MLD->getChain());
 
+  // If this is a masked load with an all ones mask, we can use a unmasked load.
+  // FIXME: Can we do this for indexed, expanding, or extending loads?
+  if (ISD::isBuildVectorAllOnes(Mask.getNode()) &&
+      MLD->isUnindexed() && !MLD->isExpandingLoad() &&
+      MLD->getExtensionType() == ISD::NON_EXTLOAD) {
+    SDValue NewLd = DAG.getLoad(N->getValueType(0), SDLoc(N), MLD->getChain(),
+                                MLD->getBasePtr(), MLD->getMemOperand());
+    return CombineTo(N, NewLd, NewLd.getValue(1));
+  }
+
   // Try transforming N to an indexed load.
   if (CombineToPreIndexedLoadStore(N) || CombineToPostIndexedLoadStore(N))
     return SDValue(N, 0);
 
   return SDValue();
 }
 
 /// A vector select of 2 constant vectors can be simplified to math/logic to

llvm/test/CodeGen/X86/masked_load.ll

 }
 
 define <4 x float> @mload_constmask_v4f32_all(<4 x float>* %addr) {
 ; SSE-LABEL: mload_constmask_v4f32_all:
 ; SSE:       ## %bb.0:
 ; SSE-NEXT:    movups (%rdi), %xmm0
 ; SSE-NEXT:    retq
 ;
-; AVX1OR2-LABEL: mload_constmask_v4f32_all:
-; AVX1OR2:       ## %bb.0:
-; AVX1OR2-NEXT:    vmovups (%rdi), %xmm0
-; AVX1OR2-NEXT:    retq
-;
-; AVX512F-LABEL: mload_constmask_v4f32_all:
-; AVX512F:       ## %bb.0:
-; AVX512F-NEXT:    movw $15, %ax
-; AVX512F-NEXT:    kmovw %eax, %k1
-; AVX512F-NEXT:    vmovups (%rdi), %zmm0 {%k1} {z}
-; AVX512F-NEXT:    ## kill: def $xmm0 killed $xmm0 killed $zmm0
-; AVX512F-NEXT:    vzeroupper
-; AVX512F-NEXT:    retq
-;
-; AVX512VL-LABEL: mload_constmask_v4f32_all:
-; AVX512VL:       ## %bb.0:
-; AVX512VL-NEXT:    kxnorw %k0, %k0, %k1
-; AVX512VL-NEXT:    vmovups (%rdi), %xmm0 {%k1} {z}
-; AVX512VL-NEXT:    retq
+; AVX-LABEL: mload_constmask_v4f32_all:
+; AVX:       ## %bb.0:
+; AVX-NEXT:    vmovups (%rdi), %xmm0
+; AVX-NEXT:    retq
   %res = call <4 x float> @llvm.masked.load.v4f32.p0v4f32(<4 x float>* %addr, i32 4, <4 x i1> <i1 true, i1 true, i1 true, i1 true>, <4 x float>undef)
   ret <4 x float> %res
 }
 
 define <2 x double> @mload_constmask_v2f64(<2 x double>* %addr, <2 x double> %dst) {
 ; SSE-LABEL: mload_constmask_v2f64:
 ; SSE:       ## %bb.0:
 ; SSE-NEXT:    movhps {{.*#+}} xmm0 = xmm0[0,1],mem[0,1]

 ; AVX1OR2-NEXT:    vmaskmovpd 96(%rdi), %ymm0, %ymm0
 ; AVX1OR2-NEXT:    vblendpd {{.*#+}} ymm3 = ymm0[0],ymm3[1],ymm0[2],ymm3[3]
 ; AVX1OR2-NEXT:    vmovups (%rdi), %ymm0
 ; AVX1OR2-NEXT:    vmovups 32(%rdi), %ymm1
 ; AVX1OR2-NEXT:    retq
 ;
 ; AVX512F-LABEL: mload_constmask_v16f64_allones_split:
 ; AVX512F:       ## %bb.0:
-; AVX512F-NEXT:    kxnorw %k0, %k0, %k1
-; AVX512F-NEXT:    vmovupd (%rdi), %zmm0 {%k1}
 ; AVX512F-NEXT:    movb $85, %al
 ; AVX512F-NEXT:    kmovw %eax, %k1
 ; AVX512F-NEXT:    vmovupd 64(%rdi), %zmm1 {%k1}
+; AVX512F-NEXT:    vmovups (%rdi), %zmm0
 ; AVX512F-NEXT:    retq
 ;
 ; AVX512VLDQ-LABEL: mload_constmask_v16f64_allones_split:
 ; AVX512VLDQ:       ## %bb.0:
-; AVX512VLDQ-NEXT:    kxnorw %k0, %k0, %k1
-; AVX512VLDQ-NEXT:    vmovupd (%rdi), %zmm0 {%k1}
 ; AVX512VLDQ-NEXT:    movb $85, %al
 ; AVX512VLDQ-NEXT:    kmovw %eax, %k1
 ; AVX512VLDQ-NEXT:    vmovupd 64(%rdi), %zmm1 {%k1}
+; AVX512VLDQ-NEXT:    vmovups (%rdi), %zmm0
 ; AVX512VLDQ-NEXT:    retq
 ;
 ; AVX512VLBW-LABEL: mload_constmask_v16f64_allones_split:
 ; AVX512VLBW:       ## %bb.0:
-; AVX512VLBW-NEXT:    kxnorw %k0, %k0, %k1
-; AVX512VLBW-NEXT:    vmovupd (%rdi), %zmm0 {%k1}
 ; AVX512VLBW-NEXT:    movb $85, %al
 ; AVX512VLBW-NEXT:    kmovd %eax, %k1
 ; AVX512VLBW-NEXT:    vmovupd 64(%rdi), %zmm1 {%k1}
+; AVX512VLBW-NEXT:    vmovups (%rdi), %zmm0
 ; AVX512VLBW-NEXT:    retq
   %res = call <16 x double> @llvm.masked.load.v16f64.p0v16f64(<16 x double>* %addr, i32 4, <16 x i1> <i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 1, i1 0, i1 1, i1 0, i1 1, i1 0, i1 1, i1 0>, <16 x double> %dst)
   ret <16 x double> %res
 }
 
 ; If the pass-through operand is undef, no blend is needed.
 
 define <4 x double> @mload_constmask_v4f64_undef_passthrough(<4 x double>* %addr) {