[llvm] [CGP] Drop poison-generating flags after hoisting (PR #90382)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Yingwei Zheng (dtcxzyw)

<details>
<summary>Changes</summary>

See the following case:
```
define i8 @<!-- -->src1(i8 %x) {
entry:
  %cmp = icmp eq i8 %x, -1
  br i1 %cmp, label %exit, label %if.then

if.then:
  %inc = add nuw nsw i8 %x, 1
  br label %exit

exit:
  %retval = phi i8 [ %inc, %if.then ], [ -1, %entry ]
  ret i8 %retval
}

define i8 @<!-- -->tgt1(i8 %x) {
entry:
  %inc = add nuw nsw i8 %x, 1
  %0 = icmp eq i8 %inc, 0
  br i1 %0, label %exit, label %if.then

if.then:                                          ; preds = %entry
  br label %exit

exit:                                             ; preds = %if.then, %entry
  %retval = phi i8 [ %inc, %if.then ], [ -1, %entry ]
  ret i8 %retval
}
```
`optimizeBranch` converts `icmp eq X, -1` into cmp to zero on RISC-V and hoists the add into the entry block. Poison-generating flags should be dropped as they don't still hold.

Proof: https://alive2.llvm.org/ce/z/sP7mvK
Fixes https://github.com/llvm/llvm-project/issues/90380

---
Full diff: https://github.com/llvm/llvm-project/pull/90382.diff


2 Files Affected:

- (modified) llvm/lib/CodeGen/CodeGenPrepare.cpp (+6-2) 
- (added) llvm/test/Transforms/CodeGenPrepare/RISCV/convert-to-eqz.ll (+54) 


``````````diff
diff --git a/llvm/lib/CodeGen/CodeGenPrepare.cpp b/llvm/lib/CodeGen/CodeGenPrepare.cpp
index 8eaf78157550ee..9e9c0a09190f37 100644
--- a/llvm/lib/CodeGen/CodeGenPrepare.cpp
+++ b/llvm/lib/CodeGen/CodeGenPrepare.cpp
@@ -8268,8 +8268,10 @@ static bool optimizeBranch(BranchInst *Branch, const TargetLowering &TLI,
     if (CmpC.isPowerOf2() && Cmp->getPredicate() == ICmpInst::ICMP_ULT &&
         match(UI, m_Shr(m_Specific(X), m_SpecificInt(CmpC.logBase2())))) {
       IRBuilder<> Builder(Branch);
-      if (UI->getParent() != Branch->getParent())
+      if (UI->getParent() != Branch->getParent()) {
         UI->moveBefore(Branch);
+        UI->dropPoisonGeneratingFlags();
+      }
       Value *NewCmp = Builder.CreateCmp(ICmpInst::ICMP_EQ, UI,
                                         ConstantInt::get(UI->getType(), 0));
       LLVM_DEBUG(dbgs() << "Converting " << *Cmp << "\n");
@@ -8281,8 +8283,10 @@ static bool optimizeBranch(BranchInst *Branch, const TargetLowering &TLI,
         (match(UI, m_Add(m_Specific(X), m_SpecificInt(-CmpC))) ||
          match(UI, m_Sub(m_Specific(X), m_SpecificInt(CmpC))))) {
       IRBuilder<> Builder(Branch);
-      if (UI->getParent() != Branch->getParent())
+      if (UI->getParent() != Branch->getParent()) {
         UI->moveBefore(Branch);
+        UI->dropPoisonGeneratingFlags();
+      }
       Value *NewCmp = Builder.CreateCmp(Cmp->getPredicate(), UI,
                                         ConstantInt::get(UI->getType(), 0));
       LLVM_DEBUG(dbgs() << "Converting " << *Cmp << "\n");
diff --git a/llvm/test/Transforms/CodeGenPrepare/RISCV/convert-to-eqz.ll b/llvm/test/Transforms/CodeGenPrepare/RISCV/convert-to-eqz.ll
new file mode 100644
index 00000000000000..a08ca34f01a9f5
--- /dev/null
+++ b/llvm/test/Transforms/CodeGenPrepare/RISCV/convert-to-eqz.ll
@@ -0,0 +1,54 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 4
+; RUN: opt -codegenprepare -S -mtriple=riscv64 < %s | FileCheck %s
+
+define i8 @hoist_add(i8 %x) {
+; CHECK-LABEL: define i8 @hoist_add(
+; CHECK-SAME: i8 [[X:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[INC:%.*]] = add i8 [[X]], 1
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp eq i8 [[INC]], 0
+; CHECK-NEXT:    br i1 [[TMP0]], label [[EXIT:%.*]], label [[IF_THEN:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    br label [[EXIT]]
+; CHECK:       exit:
+; CHECK-NEXT:    [[RETVAL:%.*]] = phi i8 [ [[INC]], [[IF_THEN]] ], [ -1, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    ret i8 [[RETVAL]]
+;
+entry:
+  %cmp = icmp eq i8 %x, -1
+  br i1 %cmp, label %exit, label %if.then
+
+if.then:
+  %inc = add nuw nsw i8 %x, 1
+  br label %exit
+
+exit:
+  %retval = phi i8 [ %inc, %if.then ], [ -1, %entry ]
+  ret i8 %retval
+}
+
+define i8 @hoist_lshr(i8 %x) {
+; CHECK-LABEL: define i8 @hoist_lshr(
+; CHECK-SAME: i8 [[X:%.*]]) {
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[INC:%.*]] = lshr i8 [[X]], 3
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp eq i8 [[INC]], 0
+; CHECK-NEXT:    br i1 [[TMP0]], label [[EXIT:%.*]], label [[IF_THEN:%.*]]
+; CHECK:       if.then:
+; CHECK-NEXT:    br label [[EXIT]]
+; CHECK:       exit:
+; CHECK-NEXT:    [[RETVAL:%.*]] = phi i8 [ [[INC]], [[IF_THEN]] ], [ -1, [[ENTRY:%.*]] ]
+; CHECK-NEXT:    ret i8 [[RETVAL]]
+;
+entry:
+  %cmp = icmp ult i8 %x, 8
+  br i1 %cmp, label %exit, label %if.then
+
+if.then:
+  %inc = lshr exact i8 %x, 3
+  br label %exit
+
+exit:
+  %retval = phi i8 [ %inc, %if.then ], [ -1, %entry ]
+  ret i8 %retval
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/90382