[IR] Relax convergence requirements on call

[Keenuts]

Before this commit, having a convergence token on a non-convergent call was considered to be an error.
This commit relaxes this requirement and allows convergence tokens to be present on non-convergent calls.

When such token is present, they have no effect as the underlying call is non-convergent.
This allows passes like DCE to strip convergent attribute from functions for which all convergent operations have been stripped. When this happens, a convergence token can still exist in the call-site, causing the verifier to complain.

Alternatives have been considered in #134863 and #134844.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-llvm-ir

Author: Nathan Gauër (Keenuts)

Changes

Before this commit, having a convergence token on a non-convergent call was considered to be an error.
This commit relaxes this requirement and allows convergence tokens to be present on non-convergent calls.

When such token is present, they have no effect as the underlying call is non-convergent.
This allows passes like DCE to strip convergent attribute from functions for which all convergent operations have been stripped. When this happens, a convergence token can still exist in the call-site, causing the verifier to complain.

Alternatives have been considered in #134863 and #134844.

---

Full diff: https://github.com/llvm/llvm-project/pull/135794.diff

5 Files Affected:

• (modified) llvm/include/llvm/IR/GenericConvergenceVerifierImpl.h (-3)
• (added) llvm/test/Transforms/ADCE/convergence.ll (+92)
• (added) llvm/test/Transforms/BDCE/convergence.ll (+62)
• (modified) llvm/test/Transforms/FunctionAttrs/convergent.ll (+29)
• (modified) llvm/test/Verifier/convergencectrl-invalid.ll (+1-1)

diff --git a/llvm/include/llvm/IR/GenericConvergenceVerifierImpl.h b/llvm/include/llvm/IR/GenericConvergenceVerifierImpl.h
index e076ac4a14162..ddcce17b77c8b 100644
--- a/llvm/include/llvm/IR/GenericConvergenceVerifierImpl.h
+++ b/llvm/include/llvm/IR/GenericConvergenceVerifierImpl.h
@@ -102,9 +102,6 @@ void GenericConvergenceVerifier<ContextT>::visit(const InstructionT &I) {
     SeenFirstConvOp = true;
 
   if (TokenDef || ConvOp != CONV_NONE) {
-    Check(isConvergent(I),
-          "Convergence control token can only be used in a convergent call.",
-          {Context.print(&I)});
     Check(ConvergenceKind != UncontrolledConvergence,
           "Cannot mix controlled and uncontrolled convergence in the same "
           "function.",
diff --git a/llvm/test/Transforms/ADCE/convergence.ll b/llvm/test/Transforms/ADCE/convergence.ll
new file mode 100644
index 0000000000000..5d7b9bd65c821
--- /dev/null
+++ b/llvm/test/Transforms/ADCE/convergence.ll
@@ -0,0 +1,92 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt %s -passes=adce -S | FileCheck %s
+
+; CHECK:      Function Attrs: convergent
+define i32 @foo(i32 %a) #0 {
+; CHECK-LABEL: define i32 @foo(
+; CHECK-SAME: i32 [[A:%.*]]) #[[ATTR0:[0-9]+]] {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    ret i32 [[A]]
+;
+entry:
+  %tk = call token @llvm.experimental.convergence.entry()
+  ret i32 %a
+}
+
+; CHECK:      Function Attrs: convergent
+define void @bar() #0 {
+; CHECK-LABEL: define void @bar(
+; CHECK-SAME: ) #[[ATTR0]] {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    ret void
+;
+entry:
+  %tk = call token @llvm.experimental.convergence.anchor()
+  ret void
+}
+
+; CHECK:      Function Attrs: convergent
+define void @baz() #0 {
+; CHECK-LABEL: define void @baz(
+; CHECK-SAME: ) #[[ATTR0]] {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    br label %[[HEADER:.*]]
+; CHECK:       [[HEADER]]:
+; CHECK-NEXT:    br i1 true, label %[[BODY:.*]], label %[[EXIT:.*]]
+; CHECK:       [[BODY]]:
+; CHECK-NEXT:    br label %[[HEADER]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    ret void
+;
+entry:
+  %tk0 = call token @llvm.experimental.convergence.entry()
+  br label %header
+
+header:
+  %tk1 = call token @llvm.experimental.convergence.loop() [ "convergencectrl"(token %tk0) ]
+  br i1 true, label %body, label %exit
+
+body:
+  br label %header
+
+exit:
+  ret void
+}
+
+define void @indirect_inner() #0 {
+; CHECK-LABEL: define void @indirect_inner(
+; CHECK-SAME: ) #[[ATTR0]] {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    ret void
+;
+entry:
+  %tk0 = call token @llvm.experimental.convergence.entry()
+  ret void
+}
+
+define void @indirect() #0 {
+; CHECK-LABEL: define void @indirect(
+; CHECK-SAME: ) #[[ATTR0]] {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    [[TK0:%.*]] = call token @llvm.experimental.convergence.entry()
+; CHECK-NEXT:    [[VAR:%.*]] = alloca ptr, align 8
+; CHECK-NEXT:    store ptr @indirect_inner, ptr [[VAR]], align 8
+; CHECK-NEXT:    [[PTR:%.*]] = load ptr, ptr [[VAR]], align 8
+; CHECK-NEXT:    call void [[PTR]]() #[[ATTR0]] [ "convergencectrl"(token [[TK0]]) ]
+; CHECK-NEXT:    ret void
+;
+entry:
+  %tk0 = call token @llvm.experimental.convergence.entry()
+  %var = alloca ptr, align 8
+  store ptr @indirect_inner, ptr %var, align 8
+  %ptr = load ptr, ptr %var, align 8
+  call void %ptr() convergent [ "convergencectrl"(token %tk0) ]
+  ret void
+}
+
+declare token @llvm.experimental.convergence.entry() #1
+declare token @llvm.experimental.convergence.anchor() #1
+declare token @llvm.experimental.convergence.loop() #1
+
+attributes #0 = { convergent }
+attributes #1 = { convergent nocallback nofree nosync nounwind willreturn memory(none) }
diff --git a/llvm/test/Transforms/BDCE/convergence.ll b/llvm/test/Transforms/BDCE/convergence.ll
new file mode 100644
index 0000000000000..51f279925b9f7
--- /dev/null
+++ b/llvm/test/Transforms/BDCE/convergence.ll
@@ -0,0 +1,62 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt %s -passes=bdce -S | FileCheck %s
+
+; CHECK:      Function Attrs: convergent
+define i32 @foo(i32 %a) #0 {
+; CHECK-LABEL: define i32 @foo(
+; CHECK-SAME: i32 [[A:%.*]]) #[[ATTR0:[0-9]+]] {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    ret i32 [[A]]
+;
+entry:
+  %tk0 = call token @llvm.experimental.convergence.entry()
+  ret i32 %a
+}
+
+; CHECK:      Function Attrs: convergent
+define void @bar() #0 {
+; CHECK-LABEL: define void @bar(
+; CHECK-SAME: ) #[[ATTR0]] {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    ret void
+;
+entry:
+  %tk0 = call token @llvm.experimental.convergence.anchor()
+  ret void
+}
+
+; CHECK:      Function Attrs: convergent
+define void @baz() #0 {
+; CHECK-LABEL: define void @baz(
+; CHECK-SAME: ) #[[ATTR0]] {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    br label %[[HEADER:.*]]
+; CHECK:       [[HEADER]]:
+; CHECK-NEXT:    br i1 true, label %[[BODY:.*]], label %[[EXIT:.*]]
+; CHECK:       [[BODY]]:
+; CHECK-NEXT:    br label %[[HEADER]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    ret void
+;
+entry:
+  %tk0 = call token @llvm.experimental.convergence.entry()
+  br label %header
+
+header:
+  %tk1 = call token @llvm.experimental.convergence.loop() [ "convergencectrl"(token %tk0) ]
+  br i1 true, label %body, label %exit
+
+body:
+  br label %header
+
+exit:
+  ret void
+}
+
+declare token @llvm.experimental.convergence.entry() #1
+declare token @llvm.experimental.convergence.anchor() #1
+declare token @llvm.experimental.convergence.loop() #1
+
+attributes #0 = { convergent }
+attributes #1 = { convergent nocallback nofree nosync nounwind willreturn memory(none) }
+
diff --git a/llvm/test/Transforms/FunctionAttrs/convergent.ll b/llvm/test/Transforms/FunctionAttrs/convergent.ll
index fe8029d39d924..409934191c3ca 100644
--- a/llvm/test/Transforms/FunctionAttrs/convergent.ll
+++ b/llvm/test/Transforms/FunctionAttrs/convergent.ll
@@ -129,3 +129,32 @@ define i32 @noopt_friend() convergent {
   %a = call i32 @noopt()
   ret i32 0
 }
+
+
+; A function which is stripped of its convergent attribute, even
+; if used in a controlled convergence call.
+; This should be OK.
+define i32 @leaf_noconvergent_used() convergent {
+; CHECK: Function Attrs: mustprogress nofree norecurse nosync nounwind willreturn memory(none)
+; CHECK-LABEL: define {{[^@]+}}@leaf_noconvergent_used
+; CHECK-SAME: () #[[ATTR0]] {
+; CHECK-NEXT:    ret i32 0
+;
+  ret i32 0
+}
+
+define i32 @nonleaf_convergent() convergent {
+; CHECK: Function Attrs: convergent mustprogress nofree norecurse nosync nounwind willreturn memory(none)
+; CHECK-LABEL: define {{[^@]+}}@nonleaf_convergent
+; CHECK-SAME: () #[[ATTR7:[0-9]+]] {
+; CHECK-NEXT:    [[TMP1:%.*]] = call token @llvm.experimental.convergence.entry()
+; CHECK-NEXT:    [[TMP2:%.*]] = call i32 @leaf_noconvergent_used() [ "convergencectrl"(token [[TMP1]]) ]
+; CHECK-NEXT:    ret i32 0
+;
+  %1 = call token @llvm.experimental.convergence.entry()
+  %2 = call i32 @leaf_noconvergent_used() [ "convergencectrl"(token %1) ]
+  ret i32 0
+}
+
+
+declare token @llvm.experimental.convergence.entry() #1
diff --git a/llvm/test/Verifier/convergencectrl-invalid.ll b/llvm/test/Verifier/convergencectrl-invalid.ll
index e1fffcd1c6033..75c33760fa0e5 100644
--- a/llvm/test/Verifier/convergencectrl-invalid.ll
+++ b/llvm/test/Verifier/convergencectrl-invalid.ll
@@ -20,7 +20,7 @@ define void @wrong_token() {
   ret void
 }
 
-; CHECK: Convergence control token can only be used in a convergent call.
+; convergence control token can be used on non-convergent calls, but it has no effect.
 ; CHECK-NEXT  call void @g(){{.*}}%t05_tok1
 define void @missing.attribute() {
   %t05_tok1 = call token @llvm.experimental.convergence.anchor()

[nikic]

Approach looks good to me, but I'll leave approval to the convergence experts :)

If it doesn't exist yet, we should probably also be stripping the convergencectrl token from non-convergent calls, to allow upwards propagation.

[ssahasra]

LGTM. But I would be happier if the tests only contained things that directly demonstrate the change.