[llvm] 7fd627a - [X86] combineSelect - `fold select(c,vtrunc(x),0)` => `X86ISD::MTRUNC(x,c)` (#212865)

[via llvm-commits]

Author: Vachan
Date: 2026-07-31T16:59:11Z
New Revision: 7fd627a749d4315260aaac6aba8221dd77c9de19

URL: https://github.com/llvm/llvm-project/commit/7fd627a749d4315260aaac6aba8221dd77c9de19
DIFF: https://github.com/llvm/llvm-project/commit/7fd627a749d4315260aaac6aba8221dd77c9de19.diff

LOG: [X86] combineSelect - `fold select(c,vtrunc(x),0)` => `X86ISD::MTRUNC(x,c)`  (#212865)

resolves #156883

Added: 
    

Modified: 
    llvm/lib/Target/X86/X86ISelLowering.cpp
    llvm/test/CodeGen/X86/avx512-trunc.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index afaa083be71bb..c78af72f11d0a 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -48713,6 +48713,19 @@ static SDValue combineSelect(SDNode *N, SelectionDAG &DAG,
     return DAG.getNode(N->getOpcode(), DL, VT, Cond, LHS, RHS);
   }
 
+  // Fold vselect(mask, vtrunc(x), 0) to vmtrunc for masked vpmovqb.
+  if (Subtarget.hasAVX512() && CondVT.isVector() &&
+      CondVT.getVectorElementType() == MVT::i1 &&
+      ISD::isBuildVectorAllZeros(RHS.getNode()) && LHS.hasOneUse()) {
+    if (LHS.getOpcode() == X86ISD::VTRUNC) {
+      SDValue TruncSrc = LHS.getOperand(0);
+      EVT TruncSrcVT = TruncSrc.getValueType();
+      if (VT == MVT::v16i8 && TruncSrcVT == MVT::v8i64) {
+        return DAG.getNode(X86ISD::VMTRUNC, DL, VT, TruncSrc, RHS, Cond);
+      }
+    }
+  }
+
   // AVX512 - Extend select to merge with target shuffle.
   // select(mask, extract_subvector(shuffle(x)), y) -->
   // extract_subvector(select(widen(mask), shuffle(x), widen(y)))

diff  --git a/llvm/test/CodeGen/X86/avx512-trunc.ll b/llvm/test/CodeGen/X86/avx512-trunc.ll
index d438e1b6a75d6..fb14d56f56005 100644
--- a/llvm/test/CodeGen/X86/avx512-trunc.ll
+++ b/llvm/test/CodeGen/X86/avx512-trunc.ll
@@ -57,9 +57,8 @@ define <8 x i8> @trunc_qb_512_maskz(<8 x i64> %i, i8 %m) #0 {
 ;
 ; SKX-LABEL: trunc_qb_512_maskz:
 ; SKX:       ## %bb.0:
-; SKX-NEXT:    vpmovqb %zmm0, %xmm0
 ; SKX-NEXT:    kmovd %edi, %k1
-; SKX-NEXT:    vmovdqu8 %xmm0, %xmm0 {%k1} {z}
+; SKX-NEXT:    vpmovqb %zmm0, %xmm0 {%k1} {z}
 ; SKX-NEXT:    vzeroupper
 ; SKX-NEXT:    retq
   %x = trunc <8 x i64> %i to <8 x i8>
@@ -1464,3 +1463,59 @@ define void @test_trunc_v2i16_v2i8(ptr %dst, ptr %src) {
   store <2 x i8> %2, ptr %dst
   ret void
 }
+
+; Test vselect(mask, vtrunc(x), 0) to vmtrunc for masked vpmovqb. PR212865
+define <8 x i8> @test_mask_vpmovqb(<8 x i64> %0, <8 x i64> %1) {
+; KNL-LABEL: test_mask_vpmovqb:
+; KNL:       ## %bb.0:
+; KNL-NEXT:    vpcmpeqq %zmm1, %zmm0, %k1
+; KNL-NEXT:    vextracti64x4 $1, %zmm0, %ymm1
+; KNL-NEXT:    vpbroadcastb {{.*#+}} ymm2 = [15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15]
+; KNL-NEXT:    vpand %ymm2, %ymm1, %ymm3
+; KNL-NEXT:    vbroadcasti128 {{.*#+}} ymm4 = [0,1,1,2,1,2,2,3,1,2,2,3,2,3,3,4,0,1,1,2,1,2,2,3,1,2,2,3,2,3,3,4]
+; KNL-NEXT:    ## ymm4 = mem[0,1,0,1]
+; KNL-NEXT:    vpshufb %ymm3, %ymm4, %ymm3
+; KNL-NEXT:    vpsrlw $4, %ymm1, %ymm1
+; KNL-NEXT:    vpand %ymm2, %ymm1, %ymm1
+; KNL-NEXT:    vpshufb %ymm1, %ymm4, %ymm1
+; KNL-NEXT:    vpaddb %ymm3, %ymm1, %ymm1
+; KNL-NEXT:    vpxor %xmm3, %xmm3, %xmm3
+; KNL-NEXT:    vpsadbw %ymm3, %ymm1, %ymm1
+; KNL-NEXT:    vpand %ymm2, %ymm0, %ymm5
+; KNL-NEXT:    vpshufb %ymm5, %ymm4, %ymm5
+; KNL-NEXT:    vpsrlw $4, %ymm0, %ymm0
+; KNL-NEXT:    vpand %ymm2, %ymm0, %ymm0
+; KNL-NEXT:    vpshufb %ymm0, %ymm4, %ymm0
+; KNL-NEXT:    vpaddb %ymm5, %ymm0, %ymm0
+; KNL-NEXT:    vpsadbw %ymm3, %ymm0, %ymm0
+; KNL-NEXT:    vinserti64x4 $1, %ymm1, %zmm0, %zmm0
+; KNL-NEXT:    vpmovqb %zmm0, %xmm0
+; KNL-NEXT:    vpternlogd {{.*#+}} zmm1 {%k1} {z} = -1
+; KNL-NEXT:    vpmovdb %zmm1, %xmm1
+; KNL-NEXT:    vpand %xmm0, %xmm1, %xmm0
+; KNL-NEXT:    vzeroupper
+; KNL-NEXT:    retq
+;
+; SKX-LABEL: test_mask_vpmovqb:
+; SKX:       ## %bb.0:
+; SKX-NEXT:    vpcmpeqq %zmm1, %zmm0, %k1
+; SKX-NEXT:    vpbroadcastb {{.*#+}} zmm1 = [15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15,15]
+; SKX-NEXT:    vpandq %zmm1, %zmm0, %zmm2
+; SKX-NEXT:    vbroadcasti32x4 {{.*#+}} zmm3 = [0,1,1,2,1,2,2,3,1,2,2,3,2,3,3,4,0,1,1,2,1,2,2,3,1,2,2,3,2,3,3,4,0,1,1,2,1,2,2,3,1,2,2,3,2,3,3,4,0,1,1,2,1,2,2,3,1,2,2,3,2,3,3,4]
+; SKX-NEXT:    ## zmm3 = mem[0,1,2,3,0,1,2,3,0,1,2,3,0,1,2,3]
+; SKX-NEXT:    vpshufb %zmm2, %zmm3, %zmm2
+; SKX-NEXT:    vpsrlw $4, %zmm0, %zmm0
+; SKX-NEXT:    vpandq %zmm1, %zmm0, %zmm0
+; SKX-NEXT:    vpshufb %zmm0, %zmm3, %zmm0
+; SKX-NEXT:    vpaddb %zmm2, %zmm0, %zmm0
+; SKX-NEXT:    vpxor %xmm1, %xmm1, %xmm1
+; SKX-NEXT:    vpsadbw %zmm1, %zmm0, %zmm0
+; SKX-NEXT:    vpmovqb %zmm0, %xmm0 {%k1} {z}
+; SKX-NEXT:    vzeroupper
+; SKX-NEXT:    retq
+  %3 = icmp eq <8 x i64> %0, %1
+  %4 = tail call range(i64 0, 65) <8 x i64> @llvm.ctpop.v8i64(<8 x i64> %0)
+  %5 = trunc nuw nsw <8 x i64> %4 to <8 x i8>
+  %6 = select <8 x i1> %3, <8 x i8> %5, <8 x i8> zeroinitializer
+  ret <8 x i8> %6
+}