[llvm] [IR] Return correct memory effects for `convergencectrl` (PR #129874)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-ir

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

`convergencectrl` doesn't imply any memory access.
Closes https://github.com/llvm/llvm-project/issues/129856.


---
Full diff: https://github.com/llvm/llvm-project/pull/129874.diff


3 Files Affected:

- (modified) llvm/lib/IR/Instructions.cpp (+5-3) 
- (added) llvm/test/Transforms/DCE/op_bundles.ll (+12) 
- (modified) llvm/test/Transforms/IRNormalizer/regression-convergence-tokens.ll (+4-4) 


``````````diff
diff --git a/llvm/lib/IR/Instructions.cpp b/llvm/lib/IR/Instructions.cpp
index b5d1bc81b9d95..d2cf0ae2c1778 100644
--- a/llvm/lib/IR/Instructions.cpp
+++ b/llvm/lib/IR/Instructions.cpp
@@ -609,15 +609,17 @@ bool CallBase::hasReadingOperandBundles() const {
   // Implementation note: this is a conservative implementation of operand
   // bundle semantics, where *any* non-assume operand bundle (other than
   // ptrauth) forces a callsite to be at least readonly.
-  return hasOperandBundlesOtherThan(
-             {LLVMContext::OB_ptrauth, LLVMContext::OB_kcfi}) &&
+  return hasOperandBundlesOtherThan({LLVMContext::OB_ptrauth,
+                                     LLVMContext::OB_kcfi,
+                                     LLVMContext::OB_convergencectrl}) &&
          getIntrinsicID() != Intrinsic::assume;
 }
 
 bool CallBase::hasClobberingOperandBundles() const {
   return hasOperandBundlesOtherThan(
              {LLVMContext::OB_deopt, LLVMContext::OB_funclet,
-              LLVMContext::OB_ptrauth, LLVMContext::OB_kcfi}) &&
+              LLVMContext::OB_ptrauth, LLVMContext::OB_kcfi,
+              LLVMContext::OB_convergencectrl}) &&
          getIntrinsicID() != Intrinsic::assume;
 }
 
diff --git a/llvm/test/Transforms/DCE/op_bundles.ll b/llvm/test/Transforms/DCE/op_bundles.ll
new file mode 100644
index 0000000000000..0d3b4db8265e8
--- /dev/null
+++ b/llvm/test/Transforms/DCE/op_bundles.ll
@@ -0,0 +1,12 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt -S < %s -passes=dce  | FileCheck %s
+
+define void @dead_readfirstlane_convergencetoken(<2 x i32> %src) convergent {
+; CHECK-LABEL: define void @dead_readfirstlane_convergencetoken(
+; CHECK-SAME: <2 x i32> [[SRC:%.*]]) #[[ATTR0:[0-9]+]] {
+; CHECK-NEXT:    ret void
+;
+  %t = tail call token @llvm.experimental.convergence.entry()
+  %vec = call <2 x i32> @llvm.amdgcn.readfirstlane.v2i32(<2 x i32> %src) [ "convergencectrl"(token %t) ]
+  ret void
+}
diff --git a/llvm/test/Transforms/IRNormalizer/regression-convergence-tokens.ll b/llvm/test/Transforms/IRNormalizer/regression-convergence-tokens.ll
index 633cf091da508..88eff971b9576 100644
--- a/llvm/test/Transforms/IRNormalizer/regression-convergence-tokens.ll
+++ b/llvm/test/Transforms/IRNormalizer/regression-convergence-tokens.ll
@@ -7,14 +7,14 @@ define i32 @nested(i32 %src) #0 {
 ; CHECK-SAME: i32 [[A0:%.*]]) #[[ATTR0:[0-9]+]] {
 ; CHECK-NEXT:  [[BB15160:.*:]]
 ; CHECK-NEXT:    [[T1:%.*]] = call token @llvm.experimental.convergence.entry()
-; CHECK-NEXT:    %"vl15001llvm.experimental.convergence.anchor()" = call token @llvm.experimental.convergence.anchor()
-; CHECK-NEXT:    %"op68297llvm.amdgcn.readfirstlane.i32([[A0]], vl15001llvm.experimental.convergence.anchor())" = call i32 @llvm.amdgcn.readfirstlane.i32(i32 [[A0]]) [ "convergencectrl"(token %"vl15001llvm.experimental.convergence.anchor()") ]
-; CHECK-NEXT:    ret i32 undef
+; CHECK-NEXT:    %"vl77672llvm.experimental.convergence.anchor()" = call token @llvm.experimental.convergence.anchor()
+; CHECK-NEXT:    %"op68297(vl77672)" = call i32 @llvm.amdgcn.readfirstlane.i32(i32 [[A0]]) [ "convergencectrl"(token %"vl77672llvm.experimental.convergence.anchor()") ]
+; CHECK-NEXT:    ret i32 %"op68297(vl77672)"
 ;
   %t1 = call token @llvm.experimental.convergence.entry()
   %t2 = call token @llvm.experimental.convergence.anchor()
   %r2 = call i32 @llvm.amdgcn.readfirstlane(i32 %src) [ "convergencectrl"(token %t2) ]
-  ret i32 undef
+  ret i32 %r2
 }
 
 ; Function Attrs: convergent nounwind readnone

``````````

</details>


https://github.com/llvm/llvm-project/pull/129874