[Sema] Sanity-check alignment requested via `__attribute__((assume_aligned(imm)))`

[lebedev.ri]

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.

[erichkeane]

LGTM. note to other reviewers, D72998 fixes the max alignment constant in this.

[lebedev.ri]

In D72994#1836495, @erichkeane wrote:

LGTM. note to other reviewers, D72998 fixes the max alignment constant in this.

Thank you for the review!

[lebedev.ri]

Rebased, NFC.

[lebedev.ri]

Seeing some bot failures, reverted this one.

clang/lib/Sema/SemaDeclAttr.cpp

if (!E->isIntegerConstantExpr(I, Context)) {
       return;
     }
 
     if (!I.isPowerOf2()) {
       Diag(AttrLoc, diag::err_alignment_not_power_of_two)
         << 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) {
     if (!OE->isValueDependent()) {
       llvm::APSInt I(64);
       if (!OE->isIntegerConstantExpr(I, Context)) {
         Diag(AttrLoc, diag::err_attribute_argument_n_type)
           << &TmpAttr << 2 << AANT_ArgumentIntegerConstant

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

 int test8(int *a, int j) {
   a = __builtin_assume_aligned(a, j); // expected-error {{must be a constant integer}}
   return a[0];
 }
 
 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
 void *test_with_fn_proto(void) __attribute__((assume_aligned(128))); // no-warning
 
 void *test_no_fn_proto() __attribute__((assume_aligned(31))); // expected-error {{requested alignment is not a power of 2}}
 void *test_no_fn_proto() __attribute__((assume_aligned(32, 73))); // no-warning
 
 void *test_no_fn_proto() __attribute__((assume_aligned)); // expected-error {{'assume_aligned' attribute takes at least 1 argument}}
 void *test_no_fn_proto() __attribute__((assume_aligned())); // expected-error {{'assume_aligned' attribute takes at least 1 argument}}
 void *test_no_fn_proto() __attribute__((assume_aligned(32, 45, 37))); // expected-error {{'assume_aligned' attribute takes no more than 2 arguments}}
 
 int pr43638(int *a) {
   a = __builtin_assume_aligned(a, 1073741824); // expected-warning {{requested alignment must be 536870912 bytes or smaller; maximum alignment assumed}}
 return a[0];
 }