[llvm] adbe9e2 - llvm-reduce: Skip setting br conditions on already constant branches (#133841)

[via llvm-commits]

Author: Matt Arsenault
Date: 2025-04-02T11:41:47+07:00
New Revision: adbe9e20bf157fda89a8896d3832dc9e3cd4351c

URL: https://github.com/llvm/llvm-project/commit/adbe9e20bf157fda89a8896d3832dc9e3cd4351c
DIFF: https://github.com/llvm/llvm-project/commit/adbe9e20bf157fda89a8896d3832dc9e3cd4351c.diff

LOG: 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.

Added: 
    llvm/test/tools/llvm-reduce/simplify-conditionals-already-constant-brcond.ll

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

Removed: 
    


################################################################################
diff  --git a/llvm/test/tools/llvm-reduce/simplify-conditionals-already-constant-brcond.ll b/llvm/test/tools/llvm-reduce/simplify-conditionals-already-constant-brcond.ll
new file mode 100644
index 0000000000000..bcb1effd1fb71
--- /dev/null
+++ b/llvm/test/tools/llvm-reduce/simplify-conditionals-already-constant-brcond.ll
@@ -0,0 +1,139 @@
+; 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
+; RUN: FileCheck -check-prefixes=RESULT-FALSE,CHECK %s < %t
+
+; 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
+; RUN: FileCheck -check-prefixes=RESULT-TRUE,CHECK %s < %t
+
+; Check that simplify-conditionals-true/false do not attempt block
+; simplification in cases that happened to already use a constant
+; true/false branch. We should not get the side effect of running
+; simplifycfg on blocks where we did not change the terminator value,
+; and not introduce unreachable code.
+
+
+; CHECK-LABEL: @br_false(
+; RESULT-FALSE: br i1 false, label %will_be_unreachable, label %exit
+; RESULT-TRUE: br label %will_be_unreachable
+define i1 @br_false(i64 %int) {
+entry:
+  %i2p = inttoptr i64 %int to ptr
+  br i1 false, label %will_be_unreachable, label %exit
+
+will_be_unreachable:
+  %load = load ptr, ptr %i2p, align 8
+  br label %for.body
+
+for.body:
+  br label %for.body
+
+exit:
+  ret i1 false
+}
+
+; CHECK-LABEL: @br_false_keep_in_unreachable(
+; CHECK: entry
+; INTERESTING: [[I2P:%.+]] = inttoptr i64 %int to ptr
+; INTERESTING: load ptr, ptr [[I2P]]
+
+; RESULT-FALSE: br i1 false, label %will_be_unreachable, label %exit
+; RESULT-TRUE: br label %will_be_unreachable
+define i1 @br_false_keep_in_unreachable(i64 %int) {
+entry:
+  br i1 false, label %will_be_unreachable, label %exit
+
+will_be_unreachable:
+  %i2p = inttoptr i64 %int to ptr
+  %load = load ptr, ptr %i2p, align 8
+  br label %for.body
+
+for.body:
+  br label %for.body
+
+exit:
+  ret i1 false
+}
+
+; CHECK-LABEL: @br_true(
+
+; RESULT-FALSE: br label %will_be_unreachable
+; RESULT-TRUE: br i1 true, label %exit, label %will_be_unreachable
+define i1 @br_true(i64 %int) {
+entry:
+  %i2p = inttoptr i64 %int to ptr
+  br i1 true, label %exit, label %will_be_unreachable
+
+will_be_unreachable:
+  %load = load ptr, ptr %i2p, align 8
+  br label %for.body
+
+for.body:
+  br label %for.body
+
+exit:
+  ret i1 false
+}
+
+; CHECK-LABEL: @br_true_keep_in_unreachable(
+; CHECK: entry:
+; INTERESTING: [[I2P:%.+]] = inttoptr i64 %int to ptr
+; INTERESTING: load ptr, ptr [[I2P]]
+
+; RESULT-FALSE: br label %will_be_unreachable
+; RESULT-TRUE: br i1 true, label %exit, label %will_be_unreachable
+define i1 @br_true_keep_in_unreachable(i64 %int) {
+entry:
+  %i2p = inttoptr i64 %int to ptr
+  br i1 true, label %exit, label %will_be_unreachable
+
+will_be_unreachable:
+  %load = load ptr, ptr %i2p, align 8
+  br label %for.body
+
+for.body:
+  br label %for.body
+
+exit:
+  ret i1 false
+}
+
+; CHECK-LABEL: @br_poison(
+; RESULT-FALSE: br label %will_be_unreachable
+; RESULT-TRUE: br label %exit
+define i1 @br_poison(i64 %int) {
+entry:
+  %i2p = inttoptr i64 %int to ptr
+  br i1 poison, label %exit, label %will_be_unreachable
+
+will_be_unreachable:
+  %load = load ptr, ptr %i2p, align 8
+  br label %for.body
+
+for.body:
+  br label %for.body
+
+exit:
+  ret i1 false
+}
+
+; CHECK-LABEL: @br_poison_keep_in_unreachable(
+; CHECK: entry:
+; INTERESTING: [[I2P:%.+]] = inttoptr i64 %int to ptr
+; INTERESTING: load ptr, ptr [[I2P]]
+
+; RESULT-FALSE: br label %will_be_unreachable
+; RESULT-TRUE: br label %exit
+define i1 @br_poison_keep_in_unreachable(i64 %int) {
+entry:
+  %i2p = inttoptr i64 %int to ptr
+  br i1 poison, label %exit, label %will_be_unreachable
+
+will_be_unreachable:
+  %load = load ptr, ptr %i2p, align 8
+  br label %for.body
+
+for.body:
+  br label %for.body
+
+exit:
+  ret i1 false
+}

diff  --git a/llvm/tools/llvm-reduce/deltas/ReduceUsingSimplifyCFG.cpp b/llvm/tools/llvm-reduce/deltas/ReduceUsingSimplifyCFG.cpp
index ec37e248da8ed..3d6b35d1895e7 100644
--- a/llvm/tools/llvm-reduce/deltas/ReduceUsingSimplifyCFG.cpp
+++ b/llvm/tools/llvm-reduce/deltas/ReduceUsingSimplifyCFG.cpp
@@ -37,17 +37,18 @@ static void reduceConditionals(Oracle &O, ReducerWorkItem &WorkItem,
   Module &M = WorkItem.getModule();
   SmallVector<BasicBlock *, 16> ToSimplify;
 
+  LLVMContext &Ctx = M.getContext();
+  ConstantInt *ConstValToSet =
+      Direction ? ConstantInt::getTrue(Ctx) : ConstantInt::getFalse(Ctx);
+
   for (auto &F : M) {
     for (auto &BB : F) {
       auto *BR = dyn_cast<BranchInst>(BB.getTerminator());
-      if (!BR || !BR->isConditional() || O.shouldKeep())
+      if (!BR || !BR->isConditional() || BR->getCondition() == ConstValToSet ||
+          O.shouldKeep())
         continue;
 
-      if (Direction)
-        BR->setCondition(ConstantInt::getTrue(BR->getContext()));
-      else
-        BR->setCondition(ConstantInt::getFalse(BR->getContext()));
-
+      BR->setCondition(ConstValToSet);
       ToSimplify.push_back(&BB);
     }
   }