[llvm] [InstCombine] ptrmask of gep for dynamic pointer aligment (PR #80002)

[via llvm-commits]

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@@ -995,6 +995,44 @@ Value *InstCombinerImpl::SimplifyDemandedUseBits(Value *V, APInt DemandedMask,
                 I, 1, (DemandedMask & ~LHSKnown.Zero).zextOrTrunc(MaskWidth)))
           return I;
 
+        // Combine:
+        // (ptrmask (getelementptr i8, ptr p, imm i), imm mask)
+        //   -> (ptrmask (getelementptr i8, ptr p, imm (i & mask)), imm mask)
+        // where only the low bits known to be zero in the pointer are changed
+        Value *InnerPtr;
+        uint64_t GEPIndex;
+        uint64_t PtrMaskImmediate;
+        if (match(I, m_Intrinsic<Intrinsic::ptrmask>(
+                         m_PtrAdd(m_Value(InnerPtr), m_ConstantInt(GEPIndex)),
+                         m_ConstantInt(PtrMaskImmediate)))) {
+
+          const unsigned trailingZeros =
+              computeKnownBits(InnerPtr, Depth + 1, I).countMinTrailingZeros();
+
+          if (trailingZeros < 64) {
+            uint64_t PointerAlignBits = (uint64_t(1) << trailingZeros) - 1;
+
+            uint64_t HighBitsGEPIndex = GEPIndex & ~PointerAlignBits;
+            uint64_t MaskedLowBitsGEPIndex =
+                GEPIndex & PointerAlignBits & PtrMaskImmediate;
+
+            uint64_t MaskedGEPIndex = HighBitsGEPIndex | MaskedLowBitsGEPIndex;
+
+            if (MaskedGEPIndex != GEPIndex) {
+              auto *GEP = cast<GetElementPtrInst>(II->getArgOperand(0));
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goldsteinn wrote:

You could make it `dyn_cast` and just fail on nullptr to be safe. But think this should be fine

https://github.com/llvm/llvm-project/pull/80002