[ubsan] Improve lowering of @llvm.allow.ubsan.check

[vitalybuka]

This fix the case, when single hot inlined callsite, prevent
checks for all other. This helps to reduce number of removed checks up to 50% (deppedes on cutoff-hot value) .

ScalarOptimizerLateEPCallback was happening during
CGSCC walk, after each inlining, but this is effectively
after inlining.

Example, order in comments:

static void overflow() {
  // 1. Inline get/set if possible
  // 2. Simplify
  // 3. LowerAllowCheckPass
  set(get() + get());
}

void test() {
  // 4. Inline
  // 5. Nothing for LowerAllowCheckPass
  overflow();
}

With this patch it will look like:

static void overflow() {
  // 1. Inline get/set if possible
  // 2. Simplify
  set(get() + get());
}

void test() {
  // 3. Inline
  // 4. Simplify
  overflow();
}

// Later, after inliner CGSCC walk complete:
// 5. LowerAllowCheckPass for `overflow`
// 6. LowerAllowCheckPass for `test`

[llvmbot]

@llvm/pr-subscribers-clang-codegen

@llvm/pr-subscribers-clang

Author: Vitaly Buka (vitalybuka)

Changes

This fix the case, when single hot inlined callsite, prevent
checks for all other. This helps to reduce number of removed checks up to 50% (deppedes on cutoff-hot value) .

ScalarOptimizerLateEPCallback was happening during
CGSCC walk, after each inlining, but this is effectively
after inlining.

Example, order in comments:

static void overflow() {
  // 1. Inline get/set if possible
  // 2. Simplify
  // 3. LowerAllowCheckPass
  set(get() + get());
}

void test() {
  // 4. Inline
  // 5. Nothing for LowerAllowCheckPass
  overflow();
}

With this patch it will look like:

static void overflow() {
  // 1. Inline get/set if possible
  // 2. Simplify
  set(get() + get());
}

void test() {
  // 3. Inline
  // 4. Simplify
  overflow();
}

// Later, after inliner CGSCC walk complete:
// 5. LowerAllowCheckPass for `overflow`
// 6. LowerAllowCheckPass for `test`

---

Full diff: https://github.com/llvm/llvm-project/pull/119013.diff

2 Files Affected:

• (modified) clang/lib/CodeGen/BackendUtil.cpp (+6-7)
• (modified) clang/test/CodeGen/allow-ubsan-check-inline.c (+2-2)

diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
index bf9b04f02e9f44..1f0e760e8ebd29 100644
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/clang/lib/CodeGen/BackendUtil.cpp
@@ -789,13 +789,12 @@ static void addSanitizers(const Triple &TargetTriple,
   }
 
   if (LowerAllowCheckPass::IsRequested()) {
-    // We can optimize after inliner, and PGO profile matching. The hook below
-    // is called at the end `buildFunctionSimplificationPipeline`, which called
-    // from `buildInlinerPipeline`, which called after profile matching.
-    PB.registerScalarOptimizerLateEPCallback(
-        [](FunctionPassManager &FPM, OptimizationLevel Level) {
-          FPM.addPass(LowerAllowCheckPass());
-        });
+    // We want to call it after inline, which is about OptimizerEarlyEPCallback.
+    PB.registerOptimizerEarlyEPCallback([](ModulePassManager &MPM,
+                                           OptimizationLevel Level,
+                                           ThinOrFullLTOPhase Phase) {
+      MPM.addPass(createModuleToFunctionPassAdaptor(LowerAllowCheckPass()));
+    });
   }
 }
 
diff --git a/clang/test/CodeGen/allow-ubsan-check-inline.c b/clang/test/CodeGen/allow-ubsan-check-inline.c
index cabe76d8034d77..1de24ab90dac0e 100644
--- a/clang/test/CodeGen/allow-ubsan-check-inline.c
+++ b/clang/test/CodeGen/allow-ubsan-check-inline.c
@@ -7,8 +7,8 @@ void set(int x);
 // We will only make decision in the `overflow` function.
 // NOINL-COUNT-1: remark: Allowed check:
 
-// FIXME: We will make decision on every inline.
-// INLINE-COUNT-1: remark: Allowed check:
+// We will make decision on every inline.
+// INLINE-COUNT-5: remark: Allowed check:
 
 static void overflow() {
   set(get() + get());