[Sema] Try 2: Attempt to perform call-size-specific __attribute__((alloc_align(param_idx))) validation

Summary:
`alloc_align` attribute takes parameter number, not the alignment itself,
so given **just** the attribute/function declaration we can't do any
sanity checking for said alignment.

However, at call site, given the actual `Expr` that is passed
into that parameter, we //might// be able to evaluate said `Expr`
as Integer Constant Expression, and perform the sanity checks.
But since there is no requirement for that argument to be an immediate,
we may fail, and that's okay.

However if we did evaluate, we should enforce the same constraints
as with `__builtin_assume_aligned()`/`__attribute__((assume_aligned(imm)))`:
said alignment is a power of two, and is not greater than our magic threshold


This was initially committed in c2a9061ac5
but reverted in 00756b1823 because of
suspicious bot failures.

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

Reviewed By: erichkeane

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D72996

clang/lib/Sema/SemaChecking.cpp

@@ -3653,6 +3653,27 @@ void Sema::checkCall(NamedDecl *FDecl, const FunctionProtoType *Proto,
     }
   }
 
+  if (FDecl && FDecl->hasAttr<AllocAlignAttr>()) {
+    auto *AA = FDecl->getAttr<AllocAlignAttr>();
+    const Expr *Arg = Args[AA->getParamIndex().getASTIndex()];
+    if (!Arg->isValueDependent()) {
+      llvm::APSInt I(64);
+      if (Arg->isIntegerConstantExpr(I, Context)) {
+        if (!I.isPowerOf2()) {
+          Diag(Arg->getExprLoc(), diag::err_alignment_not_power_of_two)
+              << Arg->getSourceRange();
+          return;
+        }
+
+        // Alignment calculations can wrap around if it's greater than 2**29.
+        unsigned MaximumAlignment = 536870912;
+        if (I > MaximumAlignment)
+          Diag(Arg->getExprLoc(), diag::warn_assume_aligned_too_great)
+              << Arg->getSourceRange() << MaximumAlignment;
+      }
+    }
+  }
+
   if (FD)
     diagnoseArgDependentDiagnoseIfAttrs(FD, ThisArg, Args, Loc);
 }

clang/test/Sema/alloc-align-attr.c

@@ -17,3 +17,15 @@ void *test_no_fn_proto(int x, int y) __attribute__((alloc_align)); // expected-e
 void *test_no_fn_proto(int x, int y) __attribute__((alloc_align())); // expected-error {{'alloc_align' attribute takes one argument}}
 void *test_no_fn_proto(int x, int y) __attribute__((alloc_align(32, 45, 37))); // expected-error {{'alloc_align' attribute takes one argument}}
 
+void *passthrought(int a) {
+  return test_ptr_alloc_align(a);
+}
+void *align16() {
+  return test_ptr_alloc_align(16);
+}
+void *align15() {
+  return test_ptr_alloc_align(15); // expected-error {{requested alignment is not a power of 2}}
+}
+void *align536870912() {
+  return test_ptr_alloc_align(1073741824); // expected-warning {{requested alignment must be 536870912 bytes or smaller; maximum alignment assumed}}
+}

clang/test/SemaCXX/alloc-align-attr.cpp

@@ -23,13 +23,19 @@ void* dependent_param_func(T param) __attribute__((alloc_align(1)));// expected-
 template <int T>
 void* illegal_align_param(int p) __attribute__((alloc_align(T))); // expected-error {{'alloc_align' attribute requires parameter 1 to be an integer constant}}
 
-void dependent_impl() {
+void dependent_impl(int align) {
   dependent_ret<int> a; // expected-note {{in instantiation of template class 'dependent_ret<int>' requested here}}
   a.Foo(1);
   a.Foo2(1);
-  dependent_ret<int*> b; 
-  a.Foo(1);
-  a.Foo2(1);
+  dependent_ret<int *> b;
+  b.Foo(1);
+  b.Foo2(1);
+  b.Foo(3);           // expected-error {{requested alignment is not a power of 2}}
+  b.Foo2(3);          // expected-error {{requested alignment is not a power of 2}}
+  b.Foo(1073741824);  // expected-warning {{requested alignment must be 536870912 bytes or smaller; maximum alignment assumed}}
+  b.Foo2(1073741824); // expected-warning {{requested alignment must be 536870912 bytes or smaller; maximum alignment assumed}}
+  b.Foo(align);
+  b.Foo2(align);
 
   dependent_param_struct<int> c; 
   c.Foo(1);