[llvm] [InstCombine] Relax the conditons of fold of `ucmp`/`scmp` into phi by allowing the phi node to use the result of `ucmp`/`scmp` more than once (PR #109593)
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Sun Sep 22 14:57:02 PDT 2024
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-llvm-transforms
Author: Volodymyr Vasylkun (Poseydon42)
<details>
<summary>Changes</summary>
This extends the optimisation implemented in #<!-- -->107769 by relaxing the condtions to make it happen. Now, the value produced by `ucmp`/`scmp` doesn't need to be one-use, but only one-user, meaning it can be present in a single phi node more than once.
---
Full diff: https://github.com/llvm/llvm-project/pull/109593.diff
2 Files Affected:
- (modified) llvm/lib/Transforms/InstCombine/InstructionCombining.cpp (+15-8)
- (modified) llvm/test/Transforms/InstCombine/phi-with-multiple-unsimplifiable-values.ll (+32)
``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index f6a0f5880cd5c7..0627f76f10891f 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1830,7 +1830,7 @@ Instruction *InstCombinerImpl::foldOpIntoPhi(Instruction &I, PHINode *PN) {
// optimization is profitable, but also to avoid creating a potentially
// invalid phi node when we have a multi-edge in the CFG.
const APInt *Ignored;
- if (isa<CmpIntrinsic>(InVal) && InVal->hasOneUse() &&
+ if (isa<CmpIntrinsic>(InVal) && InVal->hasOneUser() &&
match(&I, m_ICmp(m_Specific(PN), m_APInt(Ignored)))) {
OpsToMoveUseToIncomingBB.push_back(i);
NewPhiValues.push_back(nullptr);
@@ -1868,18 +1868,25 @@ Instruction *InstCombinerImpl::foldOpIntoPhi(Instruction &I, PHINode *PN) {
// Clone the instruction that uses the phi node and move it into the incoming
// BB because we know that the next iteration of InstCombine will simplify it.
+ SmallDenseMap<BasicBlock *, Instruction *> Clones;
for (auto OpIndex : OpsToMoveUseToIncomingBB) {
Value *Op = PN->getIncomingValue(OpIndex);
BasicBlock *OpBB = PN->getIncomingBlock(OpIndex);
- Instruction *Clone = I.clone();
- for (Use &U : Clone->operands()) {
- if (U == PN)
- U = Op;
- else
- U = U->DoPHITranslation(PN->getParent(), OpBB);
+ auto CloneIt = Clones.find(OpBB);
+ if (CloneIt == Clones.end()) {
+ Instruction *Clone = I.clone();
+ for (Use &U : Clone->operands()) {
+ if (U == PN)
+ U = Op;
+ else
+ U = U->DoPHITranslation(PN->getParent(), OpBB);
+ }
+ Clone = InsertNewInstBefore(Clone, OpBB->getTerminator()->getIterator());
+ CloneIt = Clones.insert(std::make_pair(OpBB, Clone)).first;
}
- Clone = InsertNewInstBefore(Clone, OpBB->getTerminator()->getIterator());
+
+ Instruction *Clone = CloneIt->second;
NewPhiValues[OpIndex] = Clone;
}
diff --git a/llvm/test/Transforms/InstCombine/phi-with-multiple-unsimplifiable-values.ll b/llvm/test/Transforms/InstCombine/phi-with-multiple-unsimplifiable-values.ll
index 2b75d5c5475117..cd40aa92ed4fd1 100644
--- a/llvm/test/Transforms/InstCombine/phi-with-multiple-unsimplifiable-values.ll
+++ b/llvm/test/Transforms/InstCombine/phi-with-multiple-unsimplifiable-values.ll
@@ -133,3 +133,35 @@ exit:
%r = icmp slt i8 %phi, 0
ret i1 %r
}
+
+; Same as the first transformation, but the phi node uses the result of scmp twice. This verifies that we don't clone values more than once per block
+define i1 @icmp_of_phi_of_scmp_with_constant_one_user_two_uses(i8 %c, i16 %x, i16 %y, i8 %false_val) {
+; CHECK-LABEL: define i1 @icmp_of_phi_of_scmp_with_constant_one_user_two_uses(
+; CHECK-SAME: i8 [[C:%.*]], i16 [[X:%.*]], i16 [[Y:%.*]], i8 [[FALSE_VAL:%.*]]) {
+; CHECK-NEXT: [[ENTRY:.*]]:
+; CHECK-NEXT: [[TMP0:%.*]] = icmp slt i16 [[X]], [[Y]]
+; CHECK-NEXT: switch i8 [[C]], label %[[BB_2:.*]] [
+; CHECK-NEXT: i8 0, label %[[BB:.*]]
+; CHECK-NEXT: i8 1, label %[[BB]]
+; CHECK-NEXT: ]
+; CHECK: [[BB_2]]:
+; CHECK-NEXT: br label %[[BB]]
+; CHECK: [[BB]]:
+; CHECK-NEXT: [[R:%.*]] = phi i1 [ [[TMP0]], %[[ENTRY]] ], [ [[TMP0]], %[[ENTRY]] ], [ false, %[[BB_2]] ]
+; CHECK-NEXT: ret i1 [[R]]
+;
+entry:
+ %cmp = call i8 @llvm.scmp(i16 %x, i16 %y)
+ switch i8 %c, label %bb_2 [
+ i8 0, label %bb
+ i8 1, label %bb
+ ]
+
+bb_2:
+ br label %bb
+
+bb:
+ %phi = phi i8 [ %cmp, %entry ], [ %cmp, %entry ], [ 0, %bb_2 ]
+ %r = icmp slt i8 %phi, 0
+ ret i1 %r
+}
``````````
</details>
https://github.com/llvm/llvm-project/pull/109593
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