[llvm] [SimplifyCFG] Support trunc nuw in chain of comparisons. (PR #155087)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Andreas Jonson (andjo403)

<details>
<summary>Changes</summary>

proof: https://alive2.llvm.org/ce/z/haMf3h

---
Full diff: https://github.com/llvm/llvm-project/pull/155087.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/Utils/SimplifyCFG.cpp (+9) 
- (modified) llvm/test/Transforms/SimplifyCFG/switch_create.ll (+51) 


``````````diff
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 48a99543d7083..7666d3d088fc2 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -612,6 +612,15 @@ struct ConstantComparesGatherer {
   /// If CompValue is already set, the function is expected to fail if a match
   /// is found but the value compared to is different.
   bool matchInstruction(Instruction *I, bool isEQ) {
+    Value *Val;
+    if (!isEQ && match(I, m_NUWTrunc(m_Value(Val)))) {
+      // If we already have a value for the switch, it has to match!
+      if (!setValueOnce(Val))
+        return false;
+      UsedICmps++;
+      Vals.push_back(ConstantInt::get(cast<IntegerType>(Val->getType()), 0));
+      return true;
+    }
     // If this is an icmp against a constant, handle this as one of the cases.
     ICmpInst *ICI;
     ConstantInt *C;
diff --git a/llvm/test/Transforms/SimplifyCFG/switch_create.ll b/llvm/test/Transforms/SimplifyCFG/switch_create.ll
index a06e64f74b3ac..74e9731e0a3e6 100644
--- a/llvm/test/Transforms/SimplifyCFG/switch_create.ll
+++ b/llvm/test/Transforms/SimplifyCFG/switch_create.ll
@@ -1192,3 +1192,54 @@ if.end:
   ret void
 
 }
+
+define void @chain_trunc_nuw_i1_condition(i8 %x) {
+; CHECK-LABEL: @chain_trunc_nuw_i1_condition(
+; CHECK-NEXT:    switch i8 [[X:%.*]], label [[IF_THEN:%.*]] [
+; CHECK-NEXT:      i8 4, label [[COMMON_RET:%.*]]
+; CHECK-NEXT:      i8 3, label [[COMMON_RET]]
+; CHECK-NEXT:      i8 2, label [[COMMON_RET]]
+; CHECK-NEXT:      i8 0, label [[COMMON_RET]]
+; CHECK-NEXT:    ]
+; CHECK:       common.ret:
+; CHECK-NEXT:    ret void
+; CHECK:       if.then:
+; CHECK-NEXT:    tail call void @foo1()
+; CHECK-NEXT:    br label [[COMMON_RET]]
+;
+  %add = add nsw i8 %x, -2
+  %icmp = icmp ugt i8 %add, 2
+  %trunc = trunc nuw i8 %x to i1
+  %or.cond.i = select i1 %icmp, i1 %trunc, i1 false
+  br i1 %or.cond.i, label %if.then, label %if.end
+if.then:
+  tail call void @foo1()
+  ret void
+if.end:
+  ret void
+}
+
+define void @neg_chain_trunc_i1_condition(i8 %x) {
+; CHECK-LABEL: @neg_chain_trunc_i1_condition(
+; CHECK-NEXT:    [[ADD:%.*]] = add nsw i8 [[X:%.*]], -2
+; CHECK-NEXT:    [[ICMP:%.*]] = icmp ugt i8 [[ADD]], 2
+; CHECK-NEXT:    [[TRUNC:%.*]] = trunc i8 [[X]] to i1
+; CHECK-NEXT:    [[OR_COND_I:%.*]] = select i1 [[ICMP]], i1 [[TRUNC]], i1 false
+; CHECK-NEXT:    br i1 [[OR_COND_I]], label [[IF_THEN:%.*]], label [[COMMON_RET:%.*]]
+; CHECK:       common.ret:
+; CHECK-NEXT:    ret void
+; CHECK:       if.then:
+; CHECK-NEXT:    tail call void @foo1()
+; CHECK-NEXT:    br label [[COMMON_RET]]
+;
+  %add = add nsw i8 %x, -2
+  %icmp = icmp ugt i8 %add, 2
+  %trunc = trunc i8 %x to i1
+  %or.cond.i = select i1 %icmp, i1 %trunc, i1 false
+  br i1 %or.cond.i, label %if.then, label %if.end
+if.then:
+  tail call void @foo1()
+  ret void
+if.end:
+  ret void
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/155087