[llvm] 18e6a03 - [X86][AVX] Extract SUBV_BROADCAST constant bits from just the lower subvector range (PR51281)

[Simon Pilgrim via llvm-commits]

Author: Simon Pilgrim
Date: 2021-08-06T11:21:31+01:00
New Revision: 18e6a03b1a15b2661259af15ae604b4c4850cd61

URL: https://github.com/llvm/llvm-project/commit/18e6a03b1a15b2661259af15ae604b4c4850cd61
DIFF: https://github.com/llvm/llvm-project/commit/18e6a03b1a15b2661259af15ae604b4c4850cd61.diff

LOG: [X86][AVX] Extract SUBV_BROADCAST constant bits from just the lower subvector range (PR51281)

As reported on PR51281, an internal fuzz test encountered an issue when extracting constant bits from a SUBV_BROADCAST node from a constant pool source larger than the broadcasted subvector width.

The getTargetConstantBitsFromNode was assuming that the Constant would the same size as the subvector, resulting in the incorrect packing of the per-element bits data.

This patch attempts to solve this by using the SUBV_BROADCAST node to determine the subvector width, and then ensuring we extract only the lowest bits from Constant of that subvector bitsize.

Differential Revision: https://reviews.llvm.org/D107158

Added: 
    

Modified: 
    llvm/lib/Target/X86/X86ISelLowering.cpp
    llvm/test/CodeGen/X86/pr51281.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 2efd06c8664f..72e0c383e188 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -6704,17 +6704,21 @@ static bool getTargetConstantBitsFromNode(SDValue Op, unsigned EltSizeInBits,
   if (Op.getOpcode() == X86ISD::SUBV_BROADCAST_LOAD) {
     auto *MemIntr = cast<MemIntrinsicSDNode>(Op);
     SDValue Ptr = MemIntr->getBasePtr();
+    // The source constant may be larger than the subvector broadcast,
+    // ensure we extract the correct subvector constants.
     if (const Constant *Cst = getTargetConstantFromBasePtr(Ptr)) {
       Type *CstTy = Cst->getType();
       unsigned CstSizeInBits = CstTy->getPrimitiveSizeInBits();
-      if (!CstTy->isVectorTy() || (SizeInBits % CstSizeInBits) != 0)
+      unsigned SubVecSizeInBits = MemIntr->getMemoryVT().getStoreSizeInBits();
+      if (!CstTy->isVectorTy() || (CstSizeInBits % SubVecSizeInBits) != 0 ||
+          (SizeInBits % SubVecSizeInBits) != 0)
         return false;
-      unsigned SubEltSizeInBits = CstTy->getScalarSizeInBits();
-      unsigned NumSubElts = CstSizeInBits / SubEltSizeInBits;
-      unsigned NumSubVecs = SizeInBits / CstSizeInBits;
+      unsigned CstEltSizeInBits = CstTy->getScalarSizeInBits();
+      unsigned NumSubElts = SubVecSizeInBits / CstEltSizeInBits;
+      unsigned NumSubVecs = SizeInBits / SubVecSizeInBits;
       APInt UndefSubElts(NumSubElts, 0);
       SmallVector<APInt, 64> SubEltBits(NumSubElts * NumSubVecs,
-                                        APInt(SubEltSizeInBits, 0));
+                                        APInt(CstEltSizeInBits, 0));
       for (unsigned i = 0; i != NumSubElts; ++i) {
         if (!CollectConstantBits(Cst->getAggregateElement(i), SubEltBits[i],
                                  UndefSubElts, i))

diff  --git a/llvm/test/CodeGen/X86/pr51281.ll b/llvm/test/CodeGen/X86/pr51281.ll
index 116e6d1470f8..3812f3fb448c 100644
--- a/llvm/test/CodeGen/X86/pr51281.ll
+++ b/llvm/test/CodeGen/X86/pr51281.ll
@@ -8,8 +8,8 @@
 ; CHECK-NEXT: .long   0x3eb5dbc6
 ; CHECK-NEXT: .zero   4
 ; CHECK-NEXT: .long   0x3eb5dbc6
-; CHECK-NEXT: .zero   4
-; CHECK-NEXT: .zero   4
+; CHECK-NEXT: .long   0x3eb5dbc6
+; CHECK-NEXT: .long   0x3eb5dbc6
 
 ; CHECK: .LCPI0_1:
 ; CHECK-NEXT: .long   3