[WPD]Provide branch weight for checking mode.

[mingmingl-llvm]

Checking mode aims to help diagnose and confirm undefined behavior. In most cases, source code don't cast pointers between unrelated types for virtual calls, so we expect direct calls in the frequent branch and debug trap in the unlikely branch.

This way, the overhead of checking mode is not higher than an indirect call promotion as long as the callsite doesn't run the debug trap branch.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

Author: Mingming Liu (mingmingl-llvm)

Changes

Checking mode aims to help diagnose and confirm undefined behavior. In most cases, source code don't cast pointers between unrelated types for virtual calls, so we expect direct calls in the frequent branch and debug trap in the unlikely branch.

This way, the overhead of checking mode is not higher than an indirect call promotion as long as the callsite doesn't run the debug trap branch.

---

Full diff: https://github.com/llvm/llvm-project/pull/124084.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp (+3-2)
• (modified) llvm/test/ThinLTO/X86/devirt_check.ll (+3-1)

diff --git a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
index 2f171c3c981d40..aaea67aa8fcb64 100644
--- a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
+++ b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
@@ -1225,8 +1225,9 @@ void DevirtModule::applySingleImplDevirt(VTableSlotInfo &SlotInfo,
       // perform a debug trap.
       if (DevirtCheckMode == WPDCheckMode::Trap) {
         auto *Cond = Builder.CreateICmpNE(CB.getCalledOperand(), Callee);
-        Instruction *ThenTerm =
-            SplitBlockAndInsertIfThen(Cond, &CB, /*Unreachable=*/false);
+        Instruction *ThenTerm = SplitBlockAndInsertIfThen(
+            Cond, &CB, /*Unreachable=*/false,
+            MDBuilder(M.getContext()).createUnlikelyBranchWeights());
         Builder.SetInsertPoint(ThenTerm);
         Function *TrapFn =
             Intrinsic::getOrInsertDeclaration(&M, Intrinsic::debugtrap);
diff --git a/llvm/test/ThinLTO/X86/devirt_check.ll b/llvm/test/ThinLTO/X86/devirt_check.ll
index 74f1dfd6ac012a..4a1a971177ae68 100644
--- a/llvm/test/ThinLTO/X86/devirt_check.ll
+++ b/llvm/test/ThinLTO/X86/devirt_check.ll
@@ -58,7 +58,7 @@ entry:
   ; Ensure !prof and !callees metadata for indirect call promotion removed.
   ; TRAP-NOT: prof
   ; TRAP-NOT: callees
-  ; TRAP:   br i1 %.not, label %1, label %0
+  ; TRAP:   br i1 %.not, label %1, label %0, !prof ![[UNLIKELY:[0-9]+]]
   ; TRAP: 0:
   ; TRAP:   tail call void @llvm.debugtrap()
   ; TRAP:   br label %1
@@ -89,6 +89,8 @@ entry:
 ; CHECK-LABEL:   ret i32
 ; CHECK-LABEL: }
 
+; TRAP: ![[UNLIKELY]] = !{!"branch_weights", i32 1, i32 1048575}
+
 declare i1 @llvm.type.test(i8*, metadata)
 declare void @llvm.assume(i1)
 

[llvmbot]

@llvm/pr-subscribers-lto

Author: Mingming Liu (mingmingl-llvm)

Changes

Checking mode aims to help diagnose and confirm undefined behavior. In most cases, source code don't cast pointers between unrelated types for virtual calls, so we expect direct calls in the frequent branch and debug trap in the unlikely branch.

This way, the overhead of checking mode is not higher than an indirect call promotion as long as the callsite doesn't run the debug trap branch.

---

Full diff: https://github.com/llvm/llvm-project/pull/124084.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp (+3-2)
• (modified) llvm/test/ThinLTO/X86/devirt_check.ll (+3-1)

diff --git a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
index 2f171c3c981d40..aaea67aa8fcb64 100644
--- a/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
+++ b/llvm/lib/Transforms/IPO/WholeProgramDevirt.cpp
@@ -1225,8 +1225,9 @@ void DevirtModule::applySingleImplDevirt(VTableSlotInfo &SlotInfo,
       // perform a debug trap.
       if (DevirtCheckMode == WPDCheckMode::Trap) {
         auto *Cond = Builder.CreateICmpNE(CB.getCalledOperand(), Callee);
-        Instruction *ThenTerm =
-            SplitBlockAndInsertIfThen(Cond, &CB, /*Unreachable=*/false);
+        Instruction *ThenTerm = SplitBlockAndInsertIfThen(
+            Cond, &CB, /*Unreachable=*/false,
+            MDBuilder(M.getContext()).createUnlikelyBranchWeights());
         Builder.SetInsertPoint(ThenTerm);
         Function *TrapFn =
             Intrinsic::getOrInsertDeclaration(&M, Intrinsic::debugtrap);
diff --git a/llvm/test/ThinLTO/X86/devirt_check.ll b/llvm/test/ThinLTO/X86/devirt_check.ll
index 74f1dfd6ac012a..4a1a971177ae68 100644
--- a/llvm/test/ThinLTO/X86/devirt_check.ll
+++ b/llvm/test/ThinLTO/X86/devirt_check.ll
@@ -58,7 +58,7 @@ entry:
   ; Ensure !prof and !callees metadata for indirect call promotion removed.
   ; TRAP-NOT: prof
   ; TRAP-NOT: callees
-  ; TRAP:   br i1 %.not, label %1, label %0
+  ; TRAP:   br i1 %.not, label %1, label %0, !prof ![[UNLIKELY:[0-9]+]]
   ; TRAP: 0:
   ; TRAP:   tail call void @llvm.debugtrap()
   ; TRAP:   br label %1
@@ -89,6 +89,8 @@ entry:
 ; CHECK-LABEL:   ret i32
 ; CHECK-LABEL: }
 
+; TRAP: ![[UNLIKELY]] = !{!"branch_weights", i32 1, i32 1048575}
+
 declare i1 @llvm.type.test(i8*, metadata)
 declare void @llvm.assume(i1)