[Sema] Try 2: Sanity-check alignment requested via `__attribute__((as…

…sume_aligned(imm)))`

Summary:
For `__builtin_assume_aligned()`, we do validate that the alignment
is not greater than `536870912` (D68824), but we don't do that for
`__attribute__((assume_aligned(N)))` attribute.
I suspect we should.

This was initially committed in a4cfb15
but reverted in 210f088 due to
suspicious bot failures.

Reviewers: erichkeane, aaron.ballman, hfinkel, rsmith, jdoerfert

Reviewed By: erichkeane

Subscribers: cfe-commits, llvm-commits

Tags: #llvm, #clang

Differential Revision: https://reviews.llvm.org/D72994

clang/lib/Sema/SemaDeclAttr.cpp

@@ -1625,6 +1625,12 @@ void Sema::AddAssumeAlignedAttr(Decl *D, const AttributeCommonInfo &CI, Expr *E,
         << E->getSourceRange();
       return;
     }
+
+    // Alignment calculations can wrap around if it's greater than 2**29.
+    unsigned MaximumAlignment = 536870912;
+    if (I > MaximumAlignment)
+      Diag(CI.getLoc(), diag::warn_assume_aligned_too_great)
+          << CI.getRange() << MaximumAlignment;
   }
 
   if (OE) {

clang/test/Sema/builtin-assume-aligned.c

@@ -46,6 +46,7 @@ int test8(int *a, int j) {
 void test_void_assume_aligned(void) __attribute__((assume_aligned(32))); // expected-warning {{'assume_aligned' attribute only applies to return values that are pointers}}
 int test_int_assume_aligned(void) __attribute__((assume_aligned(16))); // expected-warning {{'assume_aligned' attribute only applies to return values that are pointers}}
 void *test_ptr_assume_aligned(void) __attribute__((assume_aligned(64))); // no-warning
+void *test_ptr_assume_aligned(void) __attribute__((assume_aligned(1073741824))); // expected-warning {{requested alignment must be 536870912 bytes or smaller; maximum alignment assumed}}
 
 int j __attribute__((assume_aligned(8))); // expected-warning {{'assume_aligned' attribute only applies to Objective-C methods and functions}}
 void *test_no_fn_proto() __attribute__((assume_aligned(32))); // no-warning