[llvm] [InstCombine] Replace an integer comparison of a `phi` node with multiple `ucmp`/`scmp` operands and a constant with `phi` of individual comparisons of original intrinsic's arguments (PR #107769)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Volodymyr Vasylkun (Poseydon42)

<details>
<summary>Changes</summary>

When we have a `phi` instruction with more than one of its incoming values being a call to `ucmp` or `scmp`, which is then compared with an integer constant, we can move the comparison through the `phi` into the incoming basic blocks because we know that a comparison of `ucmp`/`scmp` with a constant will be simplified by the next iteration of InstCombine.

This should help with one of the regressions that's blocking #<!-- -->107314

There's a high chance that other similar patterns can be identified, in which case they can be easily handled by the same code by moving the check for "simplifiable" instructions into a lambda.

---
Full diff: https://github.com/llvm/llvm-project/pull/107769.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstructionCombining.cpp (+27) 
- (added) 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 8195e0539305cc..4c27359752002c 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1809,6 +1809,7 @@ Instruction *InstCombinerImpl::foldOpIntoPhi(Instruction &I, PHINode *PN) {
   // Check to see whether the instruction can be folded into each phi operand.
   // If there is one operand that does not fold, remember the BB it is in.
   SmallVector<Value *> NewPhiValues;
+  SmallVector<unsigned int> OpsToMoveUseTo;
   BasicBlock *NonSimplifiedBB = nullptr;
   Value *NonSimplifiedInVal = nullptr;
   for (unsigned i = 0; i != NumPHIValues; ++i) {
@@ -1820,6 +1821,16 @@ Instruction *InstCombinerImpl::foldOpIntoPhi(Instruction &I, PHINode *PN) {
       continue;
     }
 
+    // If the only use of phi is comparing it with a constant then we can
+    // put this comparison in the incoming BB directly after a ucmp/scmp call
+    // because we know that it will simplify to a single icmp.
+    if (isa<CmpIntrinsic>(InVal) &&
+        match(&I, m_c_ICmp(m_Specific(PN), m_Constant()))) {
+      OpsToMoveUseTo.push_back(i);
+      NewPhiValues.push_back(nullptr);
+      continue;
+    }
+
     if (NonSimplifiedBB) return nullptr;  // More than one non-simplified value.
 
     NonSimplifiedBB = InBB;
@@ -1851,6 +1862,22 @@ Instruction *InstCombinerImpl::foldOpIntoPhi(Instruction &I, PHINode *PN) {
       return nullptr;
   }
 
+  // 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.
+  for (auto OpIndex : OpsToMoveUseTo) {
+    Instruction *Clone = I.clone();
+    Value *Op = PN->getIncomingValue(OpIndex);
+    BasicBlock *OpBB = PN->getIncomingBlock(OpIndex);
+    for (Use &U : Clone->operands()) {
+      if (U == PN)
+        U = Op;
+      else
+        U = U->DoPHITranslation(PN->getParent(), OpBB);
+    }
+    Clone = InsertNewInstBefore(Clone, OpBB->getTerminator()->getIterator());
+    NewPhiValues[OpIndex] = Clone;
+  }
+
   // Okay, we can do the transformation: create the new PHI node.
   PHINode *NewPN = PHINode::Create(I.getType(), PN->getNumIncomingValues());
   InsertNewInstBefore(NewPN, PN->getIterator());
diff --git a/llvm/test/Transforms/InstCombine/phi-with-multiple-unsimplifiable-values.ll b/llvm/test/Transforms/InstCombine/phi-with-multiple-unsimplifiable-values.ll
new file mode 100644
index 00000000000000..b91d8fc6e05705
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/phi-with-multiple-unsimplifiable-values.ll
@@ -0,0 +1,32 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -passes=instcombine -S | FileCheck %s
+
+define i1 @icmp_of_phi_of_scmp_with_constant(i1 %c, i16 %x, i16 %y)
+; CHECK-LABEL: define i1 @icmp_of_phi_of_scmp_with_constant(
+; CHECK-SAME: i1 [[C:%.*]], i16 [[X:%.*]], i16 [[Y:%.*]]) {
+; CHECK-NEXT:  [[ENTRY:.*:]]
+; CHECK-NEXT:    br i1 [[C]], label %[[TRUE:.*]], label %[[FALSE:.*]]
+; CHECK:       [[TRUE]]:
+; CHECK-NEXT:    [[TMP0:%.*]] = icmp slt i16 [[X]], [[Y]]
+; CHECK-NEXT:    br label %[[EXIT:.*]]
+; CHECK:       [[FALSE]]:
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp slt i16 [[Y]], [[X]]
+; CHECK-NEXT:    br label %[[EXIT]]
+; CHECK:       [[EXIT]]:
+; CHECK-NEXT:    [[R:%.*]] = phi i1 [ [[TMP0]], %[[TRUE]] ], [ [[TMP1]], %[[FALSE]] ]
+; CHECK-NEXT:    ret i1 [[R]]
+;
+{
+entry:
+  br i1 %c, label %true, label %false
+true:
+  %cmp1 = call i8 @llvm.scmp(i16 %x, i16 %y)
+  br label %exit
+false:
+  %cmp2 = call i8 @llvm.scmp(i16 %y, i16 %x)
+  br label %exit
+exit:
+  %phi = phi i8 [%cmp1, %true], [%cmp2, %false]
+  %r = icmp slt i8 %phi, 0
+  ret i1 %r
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/107769