[llvm] [AArch64] Replace AND with LSL#2 for LDR target (#34101) (PR #89531)

[David Green via llvm-commits]

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@@ -16918,6 +16918,23 @@ bool AArch64TargetLowering::shouldFoldConstantShiftPairToMask(
     return (!C1 || !C2 || C1->getZExtValue() >= C2->getZExtValue());
   }
 
+  // We do not need to fold when this shifting used in specific load case:
+  // (ldr x, (add x, (shl (srl x, c1) 2)))
+  if (N->getOpcode() == ISD::SHL && N->hasOneUse()) {
+    if (auto C2 = dyn_cast<ConstantSDNode>(N->getOperand(1))) {
+      unsigned ShlAmt = C2->getZExtValue();
+      auto ShouldADD = *N->use_begin();
+      if (ShlAmt <= 3 && ShouldADD && ShouldADD->getOpcode() == ISD::ADD) {
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davemgreen wrote:

I don't believe that use_begin will return null if there are no uses - it would try to access an invalid iterator. Maybe add a `&& ShouldADD->hasOneUse()` or `&& !ShouldADD->use_empty()` just to make sure it exists and we don't get any weird crashes.

https://github.com/llvm/llvm-project/pull/89531