[X86] Fix assertion in AVX512 setcc combine due to invalid APInt mask width (llvm#155775)

The AVX512 setcc combine in X86ISelLowering was calling
`APInt::getLowBitsSet` with a mask width (`Len`) that could exceed the
bit width of the broadcasted scalar operand
(`BroadcastOpVT.getSizeInBits()`), leading to assertion failures.

This patch replaces `Len` with the number of defined (non-undef)
elements in the constant pool vector.

This ensures the generated mask is valid and avoids crashes when the
constant pool contains more elements than the scalar bit width can
represent.

Fixes llvm#155762

(cherry picked from commit 33b2c26)

Author: abhishek-kaushik22

llvm/lib/Target/X86/X86ISelLowering.cpp

@@ -56212,7 +56212,13 @@ static SDValue combineAVX512SetCCToKMOV(EVT VT, SDValue Op0, ISD::CondCode CC,
 
   SDValue Masked = BroadcastOp;
   if (N != 0) {
-    APInt Mask = APInt::getLowBitsSet(BroadcastOpVT.getSizeInBits(), Len);
+    unsigned BroadcastOpBitWidth = BroadcastOpVT.getSizeInBits();
+    unsigned NumDefinedElts = UndefElts.countTrailingZeros();
+
+    if (NumDefinedElts > BroadcastOpBitWidth)
+      return SDValue();
+
+    APInt Mask = APInt::getLowBitsSet(BroadcastOpBitWidth, NumDefinedElts);
     SDValue ShiftedValue = DAG.getNode(ISD::SRL, DL, BroadcastOpVT, BroadcastOp,
                                        DAG.getConstant(N, DL, BroadcastOpVT));
     Masked = DAG.getNode(ISD::AND, DL, BroadcastOpVT, ShiftedValue,

llvm/test/CodeGen/X86/kmov.ll

@@ -143,6 +143,57 @@ define <8 x i1> @invert_i8_mask_extract_8(i8 %mask) {
   ret <8 x i1> %cmp.45
 }
 
+define <8 x i1> @i8_mask_extract_7(i8 %mask) {
+; X64-AVX512-LABEL: i8_mask_extract_7:
+; X64-AVX512:       # %bb.0:
+; X64-AVX512-NEXT:    shrb %dil
+; X64-AVX512-NEXT:    movzbl %dil, %eax
+; X64-AVX512-NEXT:    kmovd %eax, %k0
+; X64-AVX512-NEXT:    vpmovm2w %k0, %xmm0
+; X64-AVX512-NEXT:    retq
+;
+; X64-KNL-LABEL: i8_mask_extract_7:
+; X64-KNL:       # %bb.0:
+; X64-KNL-NEXT:    vmovd %edi, %xmm0
+; X64-KNL-NEXT:    vpbroadcastb %xmm0, %xmm0
+; X64-KNL-NEXT:    vpbroadcastq {{.*#+}} xmm1 = [2,4,8,16,32,64,128,0,2,4,8,16,32,64,128,0]
+; X64-KNL-NEXT:    vpand %xmm1, %xmm0, %xmm0
+; X64-KNL-NEXT:    vpcmpeqb %xmm1, %xmm0, %xmm0
+; X64-KNL-NEXT:    vpmovzxbw {{.*#+}} xmm0 = xmm0[0],zero,xmm0[1],zero,xmm0[2],zero,xmm0[3],zero,xmm0[4],zero,xmm0[5],zero,xmm0[6],zero,xmm0[7],zero
+; X64-KNL-NEXT:    retq
+  %.splatinsert = insertelement <8 x i8> poison, i8 %mask, i64 0
+  %.splat = shufflevector <8 x i8> %.splatinsert, <8 x i8> poison, <8 x i32> zeroinitializer
+  %1 = and <8 x i8> %.splat, <i8 2, i8 4, i8 8, i8 16, i8 32, i8 64, i8 128, i8 poison>
+  %cmp.45 = icmp ne <8 x i8> %1, zeroinitializer
+  ret <8 x i1> %cmp.45
+}
+
+define <8 x i1> @invert_i8_mask_extract_7(i8 %mask) {
+; X64-AVX512-LABEL: invert_i8_mask_extract_7:
+; X64-AVX512:       # %bb.0:
+; X64-AVX512-NEXT:    shrb %dil
+; X64-AVX512-NEXT:    movzbl %dil, %eax
+; X64-AVX512-NEXT:    kmovd %eax, %k0
+; X64-AVX512-NEXT:    knotb %k0, %k0
+; X64-AVX512-NEXT:    vpmovm2w %k0, %xmm0
+; X64-AVX512-NEXT:    retq
+;
+; X64-KNL-LABEL: invert_i8_mask_extract_7:
+; X64-KNL:       # %bb.0:
+; X64-KNL-NEXT:    vmovd %edi, %xmm0
+; X64-KNL-NEXT:    vpbroadcastb %xmm0, %xmm0
+; X64-KNL-NEXT:    vpand {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %xmm0, %xmm0
+; X64-KNL-NEXT:    vpxor %xmm1, %xmm1, %xmm1
+; X64-KNL-NEXT:    vpcmpeqb %xmm1, %xmm0, %xmm0
+; X64-KNL-NEXT:    vpmovzxbw {{.*#+}} xmm0 = xmm0[0],zero,xmm0[1],zero,xmm0[2],zero,xmm0[3],zero,xmm0[4],zero,xmm0[5],zero,xmm0[6],zero,xmm0[7],zero
+; X64-KNL-NEXT:    retq
+  %.splatinsert = insertelement <8 x i8> poison, i8 %mask, i64 0
+  %.splat = shufflevector <8 x i8> %.splatinsert, <8 x i8> poison, <8 x i32> zeroinitializer
+  %1 = and <8 x i8> %.splat, <i8 2, i8 4, i8 8, i8 16, i8 32, i8 64, i8 128, i8 poison>
+  %cmp.45 = icmp eq <8 x i8> %1, zeroinitializer
+  ret <8 x i1> %cmp.45
+}
+
 define <4 x i1> @i16_mask_extract_4(i16 %mask) {
 ; X64-AVX512-LABEL: i16_mask_extract_4:
 ; X64-AVX512:       # %bb.0: