[llvm] c5f0b8b - [SimplifyCFG] Support trunc nuw in chain of comparisons. (#155087)

[via llvm-commits]

Author: Andreas Jonson
Date: 2025-08-31T10:14:28+02:00
New Revision: c5f0b8bf327ac86b5beafeeadaf5278afb707545

URL: https://github.com/llvm/llvm-project/commit/c5f0b8bf327ac86b5beafeeadaf5278afb707545
DIFF: https://github.com/llvm/llvm-project/commit/c5f0b8bf327ac86b5beafeeadaf5278afb707545.diff

LOG: [SimplifyCFG] Support trunc nuw in chain of comparisons. (#155087)

proof: https://alive2.llvm.org/ce/z/5PNCds

Added: 
    

Modified: 
    llvm/lib/Transforms/Utils/SimplifyCFG.cpp
    llvm/test/Transforms/SimplifyCFG/switch_create.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 93ace4d50cbdc..84a838b48eadd 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 
diff erent.
   bool matchInstruction(Instruction *I, bool isEQ) {
+    Value *Val;
+    if (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()), isEQ));
+      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..6d2a404203d02 100644
--- a/llvm/test/Transforms/SimplifyCFG/switch_create.ll
+++ b/llvm/test/Transforms/SimplifyCFG/switch_create.ll
@@ -1192,3 +1192,76 @@ if.end:
   ret void
 
 }
+
+define void @and_chain_trunc_nuw_i1_condition(i8 %x) {
+; CHECK-LABEL: @and_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
+  %and = select i1 %icmp, i1 %trunc, i1 false
+  br i1 %and, label %if.then, label %if.end
+if.then:
+  tail call void @foo1()
+  ret void
+if.end:
+  ret void
+}
+
+define void @or_chain_trunc_nuw_i1_condition(i8 %x) {
+; CHECK-LABEL: @or_chain_trunc_nuw_i1_condition(
+; CHECK-NEXT:    [[X_OFF:%.*]] = add i8 [[X:%.*]], -1
+; CHECK-NEXT:    [[SWITCH:%.*]] = icmp ult i8 [[X_OFF]], 2
+; CHECK-NEXT:    br i1 [[SWITCH]], 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]]
+;
+  %icmp = icmp eq i8 %x, 2
+  %trunc = trunc nuw i8 %x to i1
+  %or = select i1 %icmp, i1 true, i1 %trunc
+  br i1 %or, label %if.then, label %if.end
+if.then:
+  tail call void @foo1()
+  ret void
+if.end:
+  ret void
+}
+
+define void @neg_and_chain_trunc_i1_condition(i8 %x) {
+; CHECK-LABEL: @neg_and_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:    [[AND:%.*]] = select i1 [[ICMP]], i1 [[TRUNC]], i1 false
+; CHECK-NEXT:    br i1 [[AND]], 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
+  %and = select i1 %icmp, i1 %trunc, i1 false
+  br i1 %and, label %if.then, label %if.end
+if.then:
+  tail call void @foo1()
+  ret void
+if.end:
+  ret void
+}