[LoopRotate][coroutines] Avoid hoisting addresses of thread-local variables outside loops in coroutines

[alanzhao1]

Because loops in coroutines may have a co_await statement that reschedules the coroutine to another thread, we cannot cache addresses of thread-local variables obtained inside a loop by moving the computation of thoes addresses outside a loop.

Since LLVM doesn't have a model for coroutine memory accesses, this patch fixes this bug by disabling this optimization for coroutines in the same way as https://reviews.llvm.org/D135550 and https://reviews.llvm.org/D151774.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Alan Zhao (alanzhao1)

Changes

Because loops in coroutines may have a co_await statement that reschedules the coroutine to another thread, we cannot cache addresses of thread-local variables obtained inside a loop by moving the computation of thoes addresses outside a loop.

Since LLVM doesn't have a model for coroutine memory accesses, this patch fixes this bug by disabling this optimization for coroutines in the same way as https://reviews.llvm.org/D135550 and https://reviews.llvm.org/D151774.

---

Full diff: https://github.com/llvm/llvm-project/pull/81937.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/Utils/LoopRotationUtils.cpp (+9-1)
• (added) llvm/test/Transforms/LoopRotate/coroutine.ll (+34)

diff --git a/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp b/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp
index ec59a077302037..fb75f1622513df 100644
--- a/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp
+++ b/llvm/lib/Transforms/Utils/LoopRotationUtils.cpp
@@ -612,7 +612,15 @@ bool LoopRotate::rotateLoop(Loop *L, bool SimplifiedLatch) {
       // memory (without proving that the loop doesn't write).
       if (L->hasLoopInvariantOperands(Inst) && !Inst->mayReadFromMemory() &&
           !Inst->mayWriteToMemory() && !Inst->isTerminator() &&
-          !isa<DbgInfoIntrinsic>(Inst) && !isa<AllocaInst>(Inst)) {
+          !isa<DbgInfoIntrinsic>(Inst) && !isa<AllocaInst>(Inst) &&
+          // FIXME: It is not safe to cache the value of these instructions in
+          // coroutines, as the addresses of otherwise eligible variables (e.g.
+          // thread-local variables and errno) may change if the coroutine is
+          // resumed in a different thread. Therefore, we disable this
+          // optimization for correctness. However, this may block other correct
+          // optimizations. This should be reverted once we have a better model
+          // for memory access in coroutines.
+          !Inst->getFunction()->isPresplitCoroutine()) {
 
         if (LoopEntryBranch->getParent()->IsNewDbgInfoFormat &&
             !NextDbgInsts.empty()) {
diff --git a/llvm/test/Transforms/LoopRotate/coroutine.ll b/llvm/test/Transforms/LoopRotate/coroutine.ll
new file mode 100644
index 00000000000000..6dbaab2ecd7097
--- /dev/null
+++ b/llvm/test/Transforms/LoopRotate/coroutine.ll
@@ -0,0 +1,34 @@
+; RUN: opt -S -passes=loop-rotate < %s | FileCheck %s
+
+declare void @bar1()
+
+@threadlocalint = thread_local global i32 0, align 4
+
+define void @foo() #0 {
+; CHECK-LABEL: entry:
+; CHECK: call align 4 ptr @llvm.threadlocal.address.p0(ptr align 4 @threadlocalint)
+; CHECK: br {{.*}} label %cond.end
+entry:
+  br label %while.cond
+
+while.cond:
+  %1 = tail call align 4 ptr @llvm.threadlocal.address.p0(ptr align 4 @threadlocalint)
+  %2 = load i32, ptr %1, align 4
+  %cmp = icmp eq i32 %2, 0
+  br i1 %cmp, label %cond.end, label %cond.false
+
+cond.false:
+  call void @bar1()
+  unreachable
+
+; The address of threadlocalint must not be cached outside loops in presplit
+; coroutines.
+; CHECK-LABEL: cond.end:
+; CHECK: call align 4 ptr @llvm.threadlocal.address.p0(ptr align 4 @threadlocalint)
+; CHECK: br {{.*}} label %cond.end
+cond.end:
+  call void @bar1()
+  br label %while.cond
+}
+
+attributes #0 = { presplitcoroutine }

[aeubanks]

s/caching/hoisting

[ChuanqiXu9]

LGTM. Thanks.

[aeubanks]

lg with comment change

[alanzhao1]

s/caching/hoisting

Fixed the PR description, but I need to wait until this has been approved before fixing the actual commit message since that requires rewriting history and force pushing.