[llvm] [IPSCCP] Do not attempt `tryToReplaceWithConstant` if may break invariants (PR #102820)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Antonio Frighetto (antoniofrighetto)

<details>
<summary>Changes</summary>

If the call base is not marked as `willreturn`, do not attempt replacing the return value with constant, as otherwise it may lead to redundant instructions for recomputing the constant in codegen (as exhibited in https://github.com/llvm/llvm-project/issues/102791) – amongst other things.

I'm not sure why we should `tryToReplaceWithConstant` if the call base is non-removable and the function would be called nonetheless, in the first place; although just checking for `!canRemoveInstruction(CB)` leads to 36 test failures in SCCP.

Test updates / test addition to be done once confirmed this is the correct direction to pursue. 

---
Full diff: https://github.com/llvm/llvm-project/pull/102820.diff


1 Files Affected:

- (modified) llvm/lib/Transforms/Utils/SCCPSolver.cpp (+6-5) 


``````````diff
diff --git a/llvm/lib/Transforms/Utils/SCCPSolver.cpp b/llvm/lib/Transforms/Utils/SCCPSolver.cpp
index c944859cc69b8..8b4f3982ef4a8 100644
--- a/llvm/lib/Transforms/Utils/SCCPSolver.cpp
+++ b/llvm/lib/Transforms/Utils/SCCPSolver.cpp
@@ -69,12 +69,13 @@ bool SCCPSolver::tryToReplaceWithConstant(Value *V) {
   Constant *Const = getConstantOrNull(V);
   if (!Const)
     return false;
-  // Replacing `musttail` instructions with constant breaks `musttail` invariant
-  // unless the call itself can be removed.
-  // Calls with "clang.arc.attachedcall" implicitly use the return value and
-  // those uses cannot be updated with a constant.
+
+  // Replacing call ret values with constant may break some invariants, unless
+  // the call itself can be removed. Calls with "clang.arc.attachedcall"
+  // implicitly use the return value and those uses cannot be updated with a
+  // constant.
   CallBase *CB = dyn_cast<CallBase>(V);
-  if (CB && ((CB->isMustTailCall() &&
+  if (CB && (((CB->isMustTailCall() || !CB->willReturn()) &&
               !canRemoveInstruction(CB)) ||
              CB->getOperandBundle(LLVMContext::OB_clang_arc_attachedcall))) {
     Function *F = CB->getCalledFunction();

``````````

</details>


https://github.com/llvm/llvm-project/pull/102820