[IR] Value::getPointerAlignment(): handle pointer constants

Summary:
New `@test13` in `Attributor/align.ll` is the main motivation - `null` pointer
really does not limit our alignment knowledge, in fact it is fully aligned
since it has no bits set.

Here we don't special-case `null` pointer because it is somewhat controversial
to add one more place where we enforce that `null` pointer is zero,
but instead we do the more general thing of trying to perform constant-fold
of pointer constant to an integer, and perform alignment inferrment on that.

Reviewers: jdoerfert, gchatelet, courbet, sstefan1

Reviewed By: jdoerfert

Subscribers: hiraditya, arphaman, jfb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73131

1 files changed, 11 insertions, 0 deletions

diff --git a/llvm/lib/IR/Value.cpp b/llvm/lib/IR/Value.cpp
index cf9d08f..8e2d49f 100644
--- a/llvm/lib/IR/Value.cpp
+++ b/llvm/lib/IR/Value.cpp
@@ -729,6 +729,17 @@ MaybeAlign Value::getPointerAlignment(const DataLayout &DL) const {
       ConstantInt *CI = mdconst::extract<ConstantInt>(MD->getOperand(0));
       return MaybeAlign(CI->getLimitedValue());
     }
+  } else if (auto *CstPtr = dyn_cast<Constant>(this)) {
+    if (auto *CstInt = dyn_cast_or_null<ConstantInt>(ConstantExpr::getPtrToInt(
+            const_cast<Constant *>(CstPtr), DL.getIntPtrType(getType()),
+            /*OnlyIfReduced=*/true))) {
+      size_t TrailingZeros = CstInt->getValue().countTrailingZeros();
+      // 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
+                       ? uint64_t(1) << TrailingZeros
+                       : Value::MaximumAlignment);
+    }
   }
   return llvm::None;
 }