[PATCH] D66338: [CGP] Drop llvm.assume calls

[Dávid Bolvanský via Phabricator via llvm-commits]

xbolva00 created this revision.
xbolva00 added reviewers: jdoerfert, craig.topper.
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If llvm.assume expression contains side effect, Clang doesn't codegen such llvm.assume. Otherwise, if the expr is "pure/const", it may survive the whole middlend pipeline.
The problem: AFAIK llvm.assume calls are not propagated to backend. This means the backend just drops it, but fails to eliminate dead instructions related to llvm.assumes.

Should fix PR43007 and partially 43010 too.


Repository:
  rL LLVM

https://reviews.llvm.org/D66338

Files:
  lib/CodeGen/CodeGenPrepare.cpp
  test/Transforms/CodeGenPrepare/assume.ll


Index: test/Transforms/CodeGenPrepare/assume.ll
===================================================================
--- test/Transforms/CodeGenPrepare/assume.ll
+++ test/Transforms/CodeGenPrepare/assume.ll
@@ -0,0 +1,17 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -codegenprepare  -S %s | FileCheck %s
+
+declare i32 @foo()
+declare void @llvm.assume(i1)
+
+define void @bar() {
+; CHECK-LABEL: @bar(
+; CHECK-NEXT:    ret void
+;
+  %call = tail call i32 @foo() #0
+  %cmp = icmp eq i32 %call, 1
+  tail call void @llvm.assume(i1 %cmp)
+  ret void
+}
+
+attributes #0 = { nounwind readnone }
Index: lib/CodeGen/CodeGenPrepare.cpp
===================================================================
--- lib/CodeGen/CodeGenPrepare.cpp
+++ lib/CodeGen/CodeGenPrepare.cpp
@@ -6926,6 +6926,12 @@
   if (InsertedInsts.count(I))
     return false;
 
+  // Drop all llvm.assume calls
+  if (match(I, m_Intrinsic<Intrinsic::assume>())) {
+    RecursivelyDeleteTriviallyDeadInstructions(I);
+    return true;
+  }
+
   // TODO: Move into the switch on opcode below here.
   if (PHINode *P = dyn_cast<PHINode>(I)) {
     // It is possible for very late stage optimizations (such as SimplifyCFG)


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