[PATCH] D84220: [IPSCCP] Fix a bug that the "returned" attribute is not cleared when function is optimized to return undef

[Congzhe Cao via Phabricator via llvm-commits]

congzhe created this revision.
congzhe added reviewers: davide, efriedma, mssimpso.
congzhe created this object with edit policy "Administrators".
congzhe added projects: fixing bugs in llvm, LLVM.
Herald added subscribers: llvm-commits, hiraditya.

In IPSCCP when a function is optimized to return undef, it should clear the returned attribute for all its input arguments.

The bug is exposed when the value of an input argument of the function is assigned to a physical register and
because of the argument having a returned attribute, the value of this physical register will continue to be used
as the function return value right after the call instruction returns, even if the value that this register holds may 
be clobbered during the function call. This potentially results in incorrect values being used afterwards.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D84220

Files:
  llvm/lib/Transforms/Scalar/SCCP.cpp
  llvm/test/Transforms/SCCP/ipsccp-clear-returned.ll


Index: llvm/test/Transforms/SCCP/ipsccp-clear-returned.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/SCCP/ipsccp-clear-returned.ll
@@ -0,0 +1,22 @@
+; if IPSCCP determines a function returns undef,
+; then the "returned" attribute of input arguments
+; should be cleared.
+
+; RUN: opt < %s -ipsccp -S | FileCheck %s
+define i32 @main() {
+; CHECK-LABEL: @main
+entry:
+; CHECK-NEXT: entry:
+  %call = call i32 @func_return_undef(i32 1)
+; CHECK: {{call.*@func_return_undef}}
+  ret i32 0
+}
+
+define internal i32 @func_return_undef(i32 returned %arg) {
+; CHECK: {{define.*@func_return_undef}}
+; CHECK-NOT: returned
+entry:
+; CHECK-NEXT: entry:
+; CHECK-NEXT: {{ret.*undef}}
+ret i32 %arg
+}
Index: llvm/lib/Transforms/Scalar/SCCP.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/SCCP.cpp
+++ llvm/lib/Transforms/Scalar/SCCP.cpp
@@ -2038,6 +2038,8 @@
   for (unsigned i = 0, e = ReturnsToZap.size(); i != e; ++i) {
     Function *F = ReturnsToZap[i]->getParent()->getParent();
     ReturnsToZap[i]->setOperand(0, UndefValue::get(F->getReturnType()));
+    for (Argument& A : F->args())
+      F->removeParamAttr(A.getArgNo(), Attribute::Returned);
   }
 
   // If we inferred constant or undef values for globals variables, we can


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