[ubsan] Don't merge non-trap handlers if -ubsan-unique-traps or not optimized

[thurstond]

UBSan handler calls can sometimes be merged by the backend, which complicates debugging. Merging is currently disabled for UBSan traps if -ubsan-unique-traps is specified or if optimization is disabled. This patch applies the same policy to non-trap handler calls.

N.B. "-ubsan-unique-traps" becomes somewhat of a misnomer since it will now apply to non-trap handler calls as well as traps. We keep the naming for backwards compatibility.

[llvmbot]

@llvm/pr-subscribers-clang-codegen

Author: Thurston Dang (thurstond)

Changes

UBSan handler calls can sometimes be merged by the backend, which complicates debugging when a backtrace is not available (trap or min-rt). -ubsan-unique-traps prevents merging for trap mode, but not for min-rt's handler calls. This patch extends -ubsan-unique-traps to work for min-rt handlers.

It does not change the behavior when using the regular ubsan runtime; the backtrace provides enough information for debugging, hence no-merge would simply increase code size with little benefit.

N.B. "-ubsan-unique-traps" becomes somewhat of a misnomer since it will now apply to min-rt handlers as well as traps. We keep the naming for backwards compatibility.

---

Full diff: https://github.com/llvm/llvm-project/pull/119302.diff

2 Files Affected:

• (modified) clang/lib/CodeGen/CGExpr.cpp (+6)
• (modified) clang/test/CodeGen/ubsan-trap-merge.c (+1-1)

diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index 0d16408aa4de9d..9a3f573fec7725 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -3581,6 +3581,12 @@ static void emitCheckHandlerCall(CodeGenFunction &CGF,
                                llvm::AttributeList::FunctionIndex, B),
       /*Local=*/true);
   llvm::CallInst *HandlerCall = CGF.EmitNounwindRuntimeCall(Fn, FnArgs);
+  bool NoMerge = ClSanitizeDebugDeoptimization ||
+                 !CGF.CGM.getCodeGenOpts().OptimizationLevel ||
+                 (CGF.CurCodeDecl && CGF.CurCodeDecl->hasAttr<OptimizeNoneAttr>());
+  // Regular runtime provides a backtrace, making NoMerge a waste of space
+  if (NoMerge && MinimalRuntime)
+    HandlerCall->addFnAttr(llvm::Attribute::NoMerge);
   if (!MayReturn) {
     HandlerCall->setDoesNotReturn();
     CGF.Builder.CreateUnreachable();
diff --git a/clang/test/CodeGen/ubsan-trap-merge.c b/clang/test/CodeGen/ubsan-trap-merge.c
index e48681ce09377d..5c0760a539ec6a 100644
--- a/clang/test/CodeGen/ubsan-trap-merge.c
+++ b/clang/test/CodeGen/ubsan-trap-merge.c
@@ -266,4 +266,4 @@ int m(int x, int y) {
 }
 // TRAP: attributes #[[ATTR4]] = { nomerge noreturn nounwind }
 // HANDLER: attributes #[[ATTR4]] = { noreturn nounwind }
-// MINRT: attributes #[[ATTR4]] = { noreturn nounwind }
+// MINRT: attributes #[[ATTR4]] = { nomerge noreturn nounwind }

[llvmbot]

@llvm/pr-subscribers-clang

Author: Thurston Dang (thurstond)

Changes

UBSan handler calls can sometimes be merged by the backend, which complicates debugging when a backtrace is not available (trap or min-rt). -ubsan-unique-traps prevents merging for trap mode, but not for min-rt's handler calls. This patch extends -ubsan-unique-traps to work for min-rt handlers.

It does not change the behavior when using the regular ubsan runtime; the backtrace provides enough information for debugging, hence no-merge would simply increase code size with little benefit.

N.B. "-ubsan-unique-traps" becomes somewhat of a misnomer since it will now apply to min-rt handlers as well as traps. We keep the naming for backwards compatibility.

---

Full diff: https://github.com/llvm/llvm-project/pull/119302.diff

2 Files Affected:

• (modified) clang/lib/CodeGen/CGExpr.cpp (+6)
• (modified) clang/test/CodeGen/ubsan-trap-merge.c (+1-1)

diff --git a/clang/lib/CodeGen/CGExpr.cpp b/clang/lib/CodeGen/CGExpr.cpp
index 0d16408aa4de9d..9a3f573fec7725 100644
--- a/clang/lib/CodeGen/CGExpr.cpp
+++ b/clang/lib/CodeGen/CGExpr.cpp
@@ -3581,6 +3581,12 @@ static void emitCheckHandlerCall(CodeGenFunction &CGF,
                                llvm::AttributeList::FunctionIndex, B),
       /*Local=*/true);
   llvm::CallInst *HandlerCall = CGF.EmitNounwindRuntimeCall(Fn, FnArgs);
+  bool NoMerge = ClSanitizeDebugDeoptimization ||
+                 !CGF.CGM.getCodeGenOpts().OptimizationLevel ||
+                 (CGF.CurCodeDecl && CGF.CurCodeDecl->hasAttr<OptimizeNoneAttr>());
+  // Regular runtime provides a backtrace, making NoMerge a waste of space
+  if (NoMerge && MinimalRuntime)
+    HandlerCall->addFnAttr(llvm::Attribute::NoMerge);
   if (!MayReturn) {
     HandlerCall->setDoesNotReturn();
     CGF.Builder.CreateUnreachable();
diff --git a/clang/test/CodeGen/ubsan-trap-merge.c b/clang/test/CodeGen/ubsan-trap-merge.c
index e48681ce09377d..5c0760a539ec6a 100644
--- a/clang/test/CodeGen/ubsan-trap-merge.c
+++ b/clang/test/CodeGen/ubsan-trap-merge.c
@@ -266,4 +266,4 @@ int m(int x, int y) {
 }
 // TRAP: attributes #[[ATTR4]] = { nomerge noreturn nounwind }
 // HANDLER: attributes #[[ATTR4]] = { noreturn nounwind }
-// MINRT: attributes #[[ATTR4]] = { noreturn nounwind }
+// MINRT: attributes #[[ATTR4]] = { nomerge noreturn nounwind }

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