[PATCH] D155051: [AMD] ]Add sanity check that fixes bad shift operation in AMD backend

[Konrad Kusiak via Phabricator via llvm-commits]

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There is a problem with the `SILoadStoreOptimizer::dmasksCanBeCombined()` function that can lead to UB.

This boolean function decides if two masks can be combined into 1. The idea here is that the bits which are "on" in one mask, don't overlap with the "on" bits of the other. Consider an example (10 bits for simplicity):

Mask 1: `0101101000`
Mask 2: `0000000110`

Those can be combined into a single mask: `0101101110`.

To check if such an operation is possible, the code takes the mask which is greater and counts how many `0`s there are, starting from the `LSB` and stopping at the first `1`. Then, it shifts `1u` by this number and compares it with the smaller mask. The problem is that when both masks are `0`, the counter will find `32` zeroes in the first mask and will try to do a shift by `32` positions which leads to UB.

The fix is a simple sanity check, if the bigger mask is 0 or not.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D155051

Files:
  llvm/lib/Target/AMDGPU/SILoadStoreOptimizer.cpp


Index: llvm/lib/Target/AMDGPU/SILoadStoreOptimizer.cpp
===================================================================
--- llvm/lib/Target/AMDGPU/SILoadStoreOptimizer.cpp
+++ llvm/lib/Target/AMDGPU/SILoadStoreOptimizer.cpp
@@ -871,6 +871,10 @@
   unsigned MaxMask = std::max(CI.DMask, Paired.DMask);
   unsigned MinMask = std::min(CI.DMask, Paired.DMask);
 
+  // Sanity check - if both masks are 0
+  if (!MaxMask) 
+    return true;
+
   unsigned AllowedBitsForMin = llvm::countr_zero(MaxMask);
   if ((1u << AllowedBitsForMin) <= MinMask)
     return false;


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