[IR] Don't create ptrtoint expression to determine alignment (NFCI)

[nikic]

We try to determine the alignment of a constant by creating a ptrtoint expression and seeing if it folds. I believe the only case this can actually handle is where the constant is an inttoptr expression. Handle that directly instead of going through another ptrtoint expression.

I ran into this while trying to clean up our isEliminableCastPair() mess, which is going to disable ptrtoint(inttoptr) folding without DataLayout, breaking this code.

[llvmbot]

@llvm/pr-subscribers-llvm-ir

Author: Nikita Popov (nikic)

Changes

We try to determine the alignment of a constant by creating a ptrtoint expression and seeing if it folds. I believe the only case this can actually handle is where the constant is an inttoptr expression. Handle that directly instead of going through another ptrtoint expression.

I ran into this while trying to clean up our isEliminableCastPair() mess, which is going to disable ptrtoint(inttoptr) folding without DataLayout, breaking this code.

---

Full diff: https://github.com/llvm/llvm-project/pull/161364.diff

1 Files Affected:

• (modified) llvm/lib/IR/Value.cpp (+6-8)

diff --git a/llvm/lib/IR/Value.cpp b/llvm/lib/IR/Value.cpp
index 4e8f359481b81..e5e062d1cf4e2 100644
--- a/llvm/lib/IR/Value.cpp
+++ b/llvm/lib/IR/Value.cpp
@@ -1000,14 +1000,12 @@ Align Value::getPointerAlignment(const DataLayout &DL) const {
       ConstantInt *CI = mdconst::extract<ConstantInt>(MD->getOperand(0));
       return Align(CI->getLimitedValue());
     }
-  } else if (auto *CstPtr = dyn_cast<Constant>(this)) {
-    // Strip pointer casts to avoid creating unnecessary ptrtoint expression
-    // if the only "reduction" is combining a bitcast + ptrtoint.
-    CstPtr = CstPtr->stripPointerCasts();
-    if (auto *CstInt = dyn_cast_or_null<ConstantInt>(ConstantExpr::getPtrToInt(
-            const_cast<Constant *>(CstPtr), DL.getIntPtrType(getType()),
-            /*OnlyIfReduced=*/true))) {
-      size_t TrailingZeros = CstInt->getValue().countr_zero();
+  } else if (auto *CE = dyn_cast<ConstantExpr>(this)) {
+    // Determine the alignment of inttoptr(C).
+    if (CE->getOpcode() == Instruction::IntToPtr &&
+        isa<ConstantInt>(CE->getOperand(0))) {
+      ConstantInt *IntPtr = cast<ConstantInt>(CE->getOperand(0));
+      size_t TrailingZeros = IntPtr->getValue().countr_zero();
       // While the actual alignment may be large, elsewhere we have
       // an arbitrary upper alignmet limit, so let's clamp to it.
       return Align(TrailingZeros < Value::MaxAlignmentExponent

[arichardson]

I think this should be disabled for unstable pointers since the value could in theory be changed by the GC between intoptr and use but that can wait. Change LGTM