1 files changed, 33 insertions, 3 deletions

diff --git a/clang/lib/CodeGen/CGExprScalar.cpp b/clang/lib/CodeGen/CGExprScalar.cpp
index 82297664064..953ced9168c 100644
--- a/clang/lib/CodeGen/CGExprScalar.cpp
+++ b/clang/lib/CodeGen/CGExprScalar.cpp
@@ -2419,9 +2419,39 @@ ScalarExprEmitter::EmitScalarPrePostIncDec(const UnaryOperator *E, LValue LV,
 
   // Most common case by far: integer increment.
   } else if (type->isIntegerType()) {
-    // 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 (E->canOverflow() && type->isSignedIntegerOrEnumerationType()) {
+    assert((!type->isPromotableIntegerType() ||
+            (type->isSignedIntegerOrEnumerationType() ||
+             CGF.getContext()
+                 .getPromotedIntegerType(type)
+                 ->isSignedIntegerOrEnumerationType())) &&
+           "The following check expects that if we do promotion, at least one "
+           "of the types (either base or promoted) will be signed.");
+    if (CGF.SanOpts.hasOneOf(
+            SanitizerKind::ImplicitIntegerArithmeticValueChange) &&
+        type->isPromotableIntegerType()) {
+      // While `x += 1` (for `x` with width less than int) is modeled as
+      // promotion+arithmetics+demotion, and we can catch lossy demotion with
+      // ease; inc/dec with width less than int can't overflow because of
+      // promotion rules, so we omit promotion+demotion, which means that we can
+      // not catch lossy "demotion". Because we still want to catch these cases
+      // when the sanitizer is enabled, we perform the promotion, then perform
+      // the increment/decrement in the wider type, and finally
+      // perform the demotion. This will catch lossy demotions.
+
+      QualType promotedType = CGF.getContext().getPromotedIntegerType(type);
+      assert(promotedType != type && "Shouldn't promote to the same type.");
+      value = EmitScalarConversion(value, type, promotedType, E->getExprLoc());
+      Value *amt = llvm::ConstantInt::get(value->getType(), amount, true);
+      value = Builder.CreateAdd(value, amt, isInc ? "inc" : "dec");
+      // Do pass non-default ScalarConversionOpts so that sanitizer check is
+      // emitted.
+      value = EmitScalarConversion(value, promotedType, type, E->getExprLoc(),
+                                   ScalarConversionOpts(CGF.SanOpts));
+
+      // 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.
+    } else if (E->canOverflow() && type->isSignedIntegerOrEnumerationType()) {
       value = EmitIncDecConsiderOverflowBehavior(E, value, isInc);
     } else if (E->canOverflow() && type->isUnsignedIntegerType() &&
                CGF.SanOpts.has(SanitizerKind::UnsignedIntegerOverflow)) {