[llvm] 63daea8 - [SDAG] fix bug in ComputeNumSignBits of target constant

[Sanjay Patel via llvm-commits]

Author: Sanjay Patel
Date: 2022-01-26T10:22:41-05:00
New Revision: 63daea8b35cdb48d6061dc8ad72e5445b808dbce

URL: https://github.com/llvm/llvm-project/commit/63daea8b35cdb48d6061dc8ad72e5445b808dbce
DIFF: https://github.com/llvm/llvm-project/commit/63daea8b35cdb48d6061dc8ad72e5445b808dbce.diff

LOG: [SDAG] fix bug in ComputeNumSignBits of target constant

The loop below the changed line assumes that the element
width of the target constant is the same as the element
width of the loaded value, but that is not always true.

We could try harder to do some kind of min/max calc even
if the sizes don't match, but that can be another patch
if needed. This fixes #53401 (miscompile) and does not
change the motivating cases added when this analysis
was introduced:
ad298f86b7ad2a

Added: 
    

Modified: 
    llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
    llvm/test/CodeGen/X86/vselect-constants.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index e305041f7490..28363e2b9e88 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -4282,7 +4282,8 @@ unsigned SelectionDAG::ComputeNumSignBits(SDValue Op, const APInt &DemandedElts,
           // scalar cases.
           Type *CstTy = Cst->getType();
           if (CstTy->isVectorTy() &&
-              (NumElts * VTBits) == CstTy->getPrimitiveSizeInBits()) {
+              (NumElts * VTBits) == CstTy->getPrimitiveSizeInBits() &&
+              VTBits == CstTy->getScalarSizeInBits()) {
             Tmp = VTBits;
             for (unsigned i = 0; i != NumElts; ++i) {
               if (!DemandedElts[i])

diff  --git a/llvm/test/CodeGen/X86/vselect-constants.ll b/llvm/test/CodeGen/X86/vselect-constants.ll
index 8aba81824d12..f95eccae188a 100644
--- a/llvm/test/CodeGen/X86/vselect-constants.ll
+++ b/llvm/test/CodeGen/X86/vselect-constants.ll
@@ -298,6 +298,8 @@ define i32 @wrong_min_signbits(<2 x i16> %x) {
 ; AVX-NEXT:    vpcmpeqw %xmm1, %xmm0, %xmm0
 ; AVX-NEXT:    vmovdqa {{.*#+}} xmm1 = [1,0,0,0]
 ; AVX-NEXT:    vpandn %xmm1, %xmm0, %xmm0
+; AVX-NEXT:    vpsllw $15, %xmm0, %xmm0
+; AVX-NEXT:    vpsraw $15, %xmm0, %xmm0
 ; AVX-NEXT:    vpblendvb %xmm0, {{\.?LCPI[0-9]+_[0-9]+}}(%rip), %xmm1, %xmm0
 ; AVX-NEXT:    vmovd %xmm0, %eax
 ; AVX-NEXT:    retq