PR9350: increment/decrement of char (and anything else narrower than int)

can't overflow due to promotion rules; emit a wrapping add for those cases.

llvm-svn: 126816

clang/lib/CodeGen/CGExprScalar.cpp

@@ -1278,10 +1278,12 @@ ScalarExprEmitter::EmitScalarPrePostIncDec(const UnaryOperator *E, LValue LV,
 
     llvm::Value *amt = llvm::ConstantInt::get(value->getType(), amount);
 
-    if (type->isSignedIntegerType())
+    // Note that signed integer inc/dec with width less than int can't
+    // overflow because of promotion rules; we're just eliding a few steps here.
+    if (type->isSignedIntegerType() &&
+        value->getType()->getPrimitiveSizeInBits() >=
+            CGF.CGM.IntTy->getBitWidth())
       value = EmitAddConsiderOverflowBehavior(E, value, amt, isInc);
-
-    // Unsigned integer inc is always two's complement.
     else
       value = Builder.CreateAdd(value, amt, isInc ? "inc" : "dec");
   

clang/test/CodeGen/integer-overflow.c

@@ -50,11 +50,17 @@ void test1() {
   // TRAPV_HANDLER: foo(
   --a;
   
-  
   // -fwrapv should turn off inbounds for GEP's, PR9256
   extern int* P;
   ++P;
   // DEFAULT: getelementptr inbounds i32*
   // WRAPV: getelementptr i32*
   // TRAPV: getelementptr inbounds i32*
+
+  // PR9350: char increment never overflows.
+  extern volatile signed char PR9350;
+  // DEFAULT: add i8 {{.*}}, 1
+  // WRAPV: add i8 {{.*}}, 1
+  // TRAPV: add i8 {{.*}}, 1
+  ++PR9350;
 }