[flang-commits] [flang] [flang] Preserve the alignment attribute across stack/heap conversions (PR #234898)

[Caroline Newcombe via flang-commits]

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@@ -215,9 +215,15 @@ void AllocationPlacementPass::runOnOperation() {
             ? placementHook(info, basePolicy, stackBytesUsed)
             : fir::decideAllocationPlacement(info, basePolicy, stackBytesUsed);
 
-    // Account for the decision in the running stack budget.
-    if (endsUpOnStack(placement, info.isCurrentlyOnStack) && info.byteSize)
+    // Account for the decision in the running stack budget, including the
+    // worst-case padding a requested alignment may need.
+    if (endsUpOnStack(placement, info.isCurrentlyOnStack) && info.byteSize) {
       stackBytesUsed += static_cast<std::size_t>(*info.byteSize);
+      std::optional<std::int64_t> alignment =
+          alloca ? alloca.getAlignment() : allocmem.getAlignment();
+      if (alignment && *alignment > 1)
+        stackBytesUsed += static_cast<std::size_t>(*alignment - 1);
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cenewcombe wrote:

Should this padding also be considered in the placement decision above? As written, it is charged after deciding to use the stack, so it affects subsequent allocations but not whether this allocation fits. Or is that acceptable for this heuristic budget?

https://github.com/llvm/llvm-project/pull/234898