Don't apply nomerge for the first instance of each TrapBB type, per

Vitaly's feedback

Author: thurstond

clang/lib/CodeGen/CGExpr.cpp

@@ -3915,6 +3915,7 @@ void CodeGenFunction::EmitTrapCheck(llvm::Value *Checked,
                               Builder.getCurrentDebugLocation());
     Builder.CreateCondBr(Checked, Cont, TrapBB);
   } else {
+    bool NewTrapBB = (TrapBB == nullptr);
     TrapBB = createBasicBlock("trap");
     Builder.CreateCondBr(Checked, Cont, TrapBB);
     EmitBlock(TrapBB);
@@ -3928,7 +3929,8 @@ void CodeGenFunction::EmitTrapCheck(llvm::Value *Checked,
                                     CGM.getCodeGenOpts().TrapFuncName);
       TrapCall->addFnAttr(A);
     }
-    TrapCall->addFnAttr(llvm::Attribute::NoMerge);
+    if (!NewTrapBB)
+        TrapCall->addFnAttr(llvm::Attribute::NoMerge);
     TrapCall->setDoesNotReturn();
     TrapCall->setDoesNotThrow();
     Builder.CreateUnreachable();

clang/test/CodeGen/ubsan-trap-merge.c

@@ -55,7 +55,7 @@ int g(int x) {
 // CHECK-NEXT:    [[TMP3:%.*]] = extractvalue { i32, i1 } [[TMP2]], 1, !nosanitize [[META2]]
 // CHECK-NEXT:    br i1 [[TMP3]], label %[[TRAP1:.*]], label %[[CONT2:.*]], !nosanitize [[META2]]
 // CHECK:       [[TRAP1]]:
-// CHECK-NEXT:    tail call void @llvm.ubsantrap(i8 0) #[[ATTR4]], !nosanitize [[META2]]
+// CHECK-NEXT:    tail call void @llvm.ubsantrap(i8 0) #[[ATTR5:[0-9]+]], !nosanitize [[META2]]
 // CHECK-NEXT:    unreachable, !nosanitize [[META2]]
 // CHECK:       [[CONT2]]:
 // CHECK-NEXT:    [[TMP4:%.*]] = extractvalue { i32, i1 } [[TMP2]], 0, !nosanitize [[META2]]
@@ -102,4 +102,5 @@ int m(int x, int y) {
   return f(x) + g(y);
 }
 //.
-// CHECK: attributes #[[ATTR4]] = { nomerge noreturn nounwind }
+// CHECK: attributes #[[ATTR4]] = { noreturn nounwind }
+// CHECK: attributes #[[ATTR5]] = { nomerge noreturn nounwind }