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llvm-reduce: Skip setting br conditions on already constant branches (#133841)
If we are trying to simplify branch conditions to true, ignore branches already set to a constant true. If we are simplifying to constant false, ignore the already constant false cases. This saves steps in this edge case, and avoids the side effect of running simplifycfg on blocks we did not intend to modify.
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‎llvm/test/tools/llvm-reduce/simplify-conditionals-already-constant-brcond.ll

Lines changed: 139 additions & 0 deletions
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@@ -0,0 +1,139 @@
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; RUN: llvm-reduce --abort-on-invalid-reduction --delta-passes=simplify-conditionals-false --test FileCheck --test-arg --check-prefixes=INTERESTING --test-arg %s --test-arg --input-file %s -o %t
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; RUN: FileCheck -check-prefixes=RESULT-FALSE,CHECK %s < %t
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; RUN: llvm-reduce --abort-on-invalid-reduction --delta-passes=simplify-conditionals-true --test FileCheck --test-arg --check-prefixes=INTERESTING --test-arg %s --test-arg --input-file %s -o %t
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; RUN: FileCheck -check-prefixes=RESULT-TRUE,CHECK %s < %t
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; Check that simplify-conditionals-true/false do not attempt block
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; simplification in cases that happened to already use a constant
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; true/false branch. We should not get the side effect of running
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; simplifycfg on blocks where we did not change the terminator value,
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; and not introduce unreachable code.
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; CHECK-LABEL: @br_false(
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; RESULT-FALSE: br i1 false, label %will_be_unreachable, label %exit
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; RESULT-TRUE: br label %will_be_unreachable
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define i1 @br_false(i64 %int) {
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entry:
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%i2p = inttoptr i64 %int to ptr
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br i1 false, label %will_be_unreachable, label %exit
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will_be_unreachable:
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%load = load ptr, ptr %i2p, align 8
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br label %for.body
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for.body:
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br label %for.body
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exit:
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ret i1 false
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}
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; CHECK-LABEL: @br_false_keep_in_unreachable(
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; CHECK: entry
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; INTERESTING: [[I2P:%.+]] = inttoptr i64 %int to ptr
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; INTERESTING: load ptr, ptr [[I2P]]
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; RESULT-FALSE: br i1 false, label %will_be_unreachable, label %exit
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; RESULT-TRUE: br label %will_be_unreachable
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define i1 @br_false_keep_in_unreachable(i64 %int) {
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entry:
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br i1 false, label %will_be_unreachable, label %exit
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will_be_unreachable:
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%i2p = inttoptr i64 %int to ptr
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%load = load ptr, ptr %i2p, align 8
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br label %for.body
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for.body:
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br label %for.body
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exit:
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ret i1 false
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}
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; CHECK-LABEL: @br_true(
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; RESULT-FALSE: br label %will_be_unreachable
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; RESULT-TRUE: br i1 true, label %exit, label %will_be_unreachable
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define i1 @br_true(i64 %int) {
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entry:
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%i2p = inttoptr i64 %int to ptr
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br i1 true, label %exit, label %will_be_unreachable
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will_be_unreachable:
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%load = load ptr, ptr %i2p, align 8
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br label %for.body
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for.body:
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br label %for.body
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exit:
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ret i1 false
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}
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; CHECK-LABEL: @br_true_keep_in_unreachable(
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; CHECK: entry:
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; INTERESTING: [[I2P:%.+]] = inttoptr i64 %int to ptr
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; INTERESTING: load ptr, ptr [[I2P]]
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; RESULT-FALSE: br label %will_be_unreachable
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; RESULT-TRUE: br i1 true, label %exit, label %will_be_unreachable
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define i1 @br_true_keep_in_unreachable(i64 %int) {
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entry:
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%i2p = inttoptr i64 %int to ptr
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br i1 true, label %exit, label %will_be_unreachable
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will_be_unreachable:
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%load = load ptr, ptr %i2p, align 8
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br label %for.body
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for.body:
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br label %for.body
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exit:
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ret i1 false
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}
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; CHECK-LABEL: @br_poison(
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; RESULT-FALSE: br label %will_be_unreachable
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; RESULT-TRUE: br label %exit
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define i1 @br_poison(i64 %int) {
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entry:
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%i2p = inttoptr i64 %int to ptr
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br i1 poison, label %exit, label %will_be_unreachable
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will_be_unreachable:
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%load = load ptr, ptr %i2p, align 8
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br label %for.body
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for.body:
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br label %for.body
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exit:
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ret i1 false
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}
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; CHECK-LABEL: @br_poison_keep_in_unreachable(
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; CHECK: entry:
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; INTERESTING: [[I2P:%.+]] = inttoptr i64 %int to ptr
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; INTERESTING: load ptr, ptr [[I2P]]
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; RESULT-FALSE: br label %will_be_unreachable
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; RESULT-TRUE: br label %exit
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define i1 @br_poison_keep_in_unreachable(i64 %int) {
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entry:
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%i2p = inttoptr i64 %int to ptr
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br i1 poison, label %exit, label %will_be_unreachable
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will_be_unreachable:
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%load = load ptr, ptr %i2p, align 8
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br label %for.body
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for.body:
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br label %for.body
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exit:
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ret i1 false
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}

‎llvm/tools/llvm-reduce/deltas/ReduceUsingSimplifyCFG.cpp

Lines changed: 7 additions & 6 deletions
Original file line numberDiff line numberDiff line change
@@ -37,17 +37,18 @@ static void reduceConditionals(Oracle &O, ReducerWorkItem &WorkItem,
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Module &M = WorkItem.getModule();
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SmallVector<BasicBlock *, 16> ToSimplify;
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LLVMContext &Ctx = M.getContext();
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ConstantInt *ConstValToSet =
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Direction ? ConstantInt::getTrue(Ctx) : ConstantInt::getFalse(Ctx);
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for (auto &F : M) {
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for (auto &BB : F) {
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auto *BR = dyn_cast<BranchInst>(BB.getTerminator());
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if (!BR || !BR->isConditional() || O.shouldKeep())
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if (!BR || !BR->isConditional() || BR->getCondition() == ConstValToSet ||
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O.shouldKeep())
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continue;
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if (Direction)
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BR->setCondition(ConstantInt::getTrue(BR->getContext()));
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else
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BR->setCondition(ConstantInt::getFalse(BR->getContext()));
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BR->setCondition(ConstValToSet);
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ToSimplify.push_back(&BB);
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}
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}

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