[SimplifyCFG] Add command-line option to disable the pass

[madhur13490]

The option is useful for some of the experiments we're doing. I don't see any harm in having such option and the ability to disable the pass.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Madhur Amilkanthwar (madhur13490)

Changes

The option is useful for some of the experiments we're doing. I don't see any harm in having such option and the ability to disable the pass.

---

Full diff: https://github.com/llvm/llvm-project/pull/130965.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp (+7)
• (added) llvm/test/Transforms/SimplifyCFG/disable-simplify-cfg.ll (+13)

diff --git a/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp b/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp
index 4e437e9abeb43..74e4dd2bd991a 100644
--- a/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp
+++ b/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp
@@ -85,6 +85,10 @@ static cl::opt<bool> UserSpeculateUnpredictables(
     "speculate-unpredictables", cl::Hidden, cl::init(false),
     cl::desc("Speculate unpredictable branches (default = false)"));
 
+static cl::opt<bool> DisableSimplifyCFG(
+    "disable-simplify-cfg", cl::Hidden, cl::init(false),
+    cl::desc("Disable simplify cfg"));
+
 STATISTIC(NumSimpl, "Number of blocks simplified");
 
 static bool
@@ -307,6 +311,9 @@ static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
           (DT && DT->verify(DominatorTree::VerificationLevel::Full))) &&
          "Original domtree is invalid?");
 
+  if (DisableSimplifyCFG)
+    return false;
+
   bool Changed = simplifyFunctionCFGImpl(F, TTI, DT, Options);
 
   assert((!RequireAndPreserveDomTree ||
diff --git a/llvm/test/Transforms/SimplifyCFG/disable-simplify-cfg.ll b/llvm/test/Transforms/SimplifyCFG/disable-simplify-cfg.ll
new file mode 100644
index 0000000000000..f5cef9314e283
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/disable-simplify-cfg.ll
@@ -0,0 +1,13 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 5
+; RUN: opt < %s -passes=simplifycfg -disable-simplify-cfg=true -S | FileCheck %s
+
+define void @test1() {
+; CHECK-LABEL: define void @test1() {
+; CHECK-NEXT:    br label %[[BB1:.*]]
+; CHECK:       [[BB1]]:
+; CHECK-NEXT:    ret void
+;
+  br label %1
+  ret void
+}
+

[github-actions]

⚠️ C/C++ code formatter, clang-format found issues in your code. ⚠️

You can test this locally with the following command:

git-clang-format --diff 418e07b7e679246a3a4ab9a5a8c119eb4ba4623d 664881c9ee2704e6b3a322b75409eefe686435b2 --extensions cpp -- llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp

View the diff from clang-format here.

diff --git a/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp b/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp
index 74e4dd2bd9..01e73a294f 100644
--- a/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp
+++ b/llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp
@@ -85,9 +85,9 @@ static cl::opt<bool> UserSpeculateUnpredictables(
     "speculate-unpredictables", cl::Hidden, cl::init(false),
     cl::desc("Speculate unpredictable branches (default = false)"));
 
-static cl::opt<bool> DisableSimplifyCFG(
-    "disable-simplify-cfg", cl::Hidden, cl::init(false),
-    cl::desc("Disable simplify cfg"));
+static cl::opt<bool> DisableSimplifyCFG("disable-simplify-cfg", cl::Hidden,
+                                        cl::init(false),
+                                        cl::desc("Disable simplify cfg"));
 
 STATISTIC(NumSimpl, "Number of blocks simplified");
 

[nikic]

The option is useful for some of the experiments we're doing. I don't see any harm in having such option and the ability to disable the pass.

Could you please provide some more details what this is useful for?

[madhur13490]

The option is useful for some of the experiments we're doing. I don't see any harm in having such option and the ability to disable the pass.

Could you please provide some more details what this is useful for?

This pass aggressively converts if-else to selects. Disabling the pass and thus select generation for some of the internal benchmarks would help us narrow down the issues.

[madhur13490]

@nikic Gentle ping!

[nikic]

I don't think adding an option to disable simplifycfg makes a lot of sense. This is a core pass that does a lot more than just if conversion.

This seems like something that should be kept downstream for experimentation purposes.

[madhur13490]

@nikic Thanks. I haven't looked in the code enough but do you think disabling just the if-conversion is possible?