[clang codegen] Use IR "align" attribute for static array arguments.

[efriedma]

Summary

Without the "align" attribute, marking the argument dereferenceable is basically useless. See also D80166.

Fixes https://bugs.llvm.org/show_bug.cgi?id=46876 .

[efriedma]

Ping

[rjmccall]

clang/lib/CodeGen/CGCall.cpp

2503

Is it reasonable to assume that this is an array of complete objects, or do we need to worry about someone doing e.g.

class A : virtual HighlyAligned { char buffer[16]; ];

void foo(A[static 1]);

This isn't completely disabled in C++ mode, is it?

[efriedma]

clang/lib/CodeGen/CGCall.cpp

2503

It is disabled in C++: "error: static array size is a C99 feature, not permitted in C++",

[rjmccall]

Okay. LGTM, then.

clang/lib/CodeGen/CGCall.cpp

case ABIArgInfo::Direct: {
           if (const auto *ArrTy =
               getContext().getAsConstantArrayType(OTy)) {
             // A C99 array parameter declaration with the static keyword also
             // indicates dereferenceability, and if the size is constant we can
             // use the dereferenceable attribute (which requires the size in
             // bytes).
             if (ArrTy->getSizeModifier() == ArrayType::Static) {
               QualType ETy = ArrTy->getElementType();
+              llvm::Align Alignment =
+                  CGM.getNaturalTypeAlignment(ETy).getAsAlign();
+              AI->addAttrs(llvm::AttrBuilder().addAlignmentAttr(Alignment));
               uint64_t ArrSize = ArrTy->getSize().getZExtValue();
               if (!ETy->isIncompleteType() && ETy->isConstantSizeType() &&
                   ArrSize) {
                 llvm::AttrBuilder Attrs;
                 Attrs.addDereferenceableAttr(
                     getContext().getTypeSizeInChars(ETy).getQuantity() *
                     ArrSize);
                 AI->addAttrs(Attrs);
               } else if (getContext().getTargetInfo().getNullPointerValue(
                              ETy.getAddressSpace()) == 0 &&
                          !CGM.getCodeGenOpts().NullPointerIsValid) {
                 AI->addAttr(llvm::Attribute::NonNull);
               }
             }
           } else if (const auto *ArrTy =
                      getContext().getAsVariableArrayType(OTy)) {
             // For C99 VLAs with the static keyword, we don't know the size so
             // we can't use the dereferenceable attribute, but in addrspace(0)
             // we know that it must be nonnull.
-            if (ArrTy->getSizeModifier() == VariableArrayType::Static &&
-                !getContext().getTargetAddressSpace(ArrTy->getElementType()) &&
+            if (ArrTy->getSizeModifier() == VariableArrayType::Static) {
+              QualType ETy = ArrTy->getElementType();
+              llvm::Align Alignment =
+                  CGM.getNaturalTypeAlignment(ETy).getAsAlign();
+              AI->addAttrs(llvm::AttrBuilder().addAlignmentAttr(Alignment));
+              if (!getContext().getTargetAddressSpace(ETy) &&
                   !CGM.getCodeGenOpts().NullPointerIsValid)
                 AI->addAttr(llvm::Attribute::NonNull);
             }
+          }
 
           // Set `align` attribute if any.
           const auto *AVAttr = PVD->getAttr<AlignValueAttr>();
           if (!AVAttr)
             if (const auto *TOTy = dyn_cast<TypedefType>(OTy))
               AVAttr = TOTy->getDecl()->getAttr<AlignValueAttr>();
           if (AVAttr && !SanOpts.has(SanitizerKind::Alignment)) {
             // If alignment-assumption sanitizer is enabled, we do *not* add

clang/test/CodeGen/vla.c

 void test7(int a[b(0)]) {
   // CHECK-LABEL: define void @test7(
   // CHECK: call i32 @b(i8* null)
 }
 
 // Make sure we emit dereferenceable or nonnull when the static keyword is
 // provided.
 void test8(int a[static 3]) { }
-// CHECK: define void @test8(i32* dereferenceable(12) %a)
+// CHECK: define void @test8(i32* align 4 dereferenceable(12) %a)
 
 void test9(int n, int a[static n]) { }
-// NULL-INVALID: define void @test9(i32 %n, i32* nonnull %a)
-// NULL-VALID: define void @test9(i32 %n, i32* %a)
+// NULL-INVALID: define void @test9(i32 %n, i32* nonnull align 4 %a)
+// NULL-VALID: define void @test9(i32 %n, i32* align 4 %a)
 
 // Make sure a zero-sized static array extent is still required to be nonnull.
 void test10(int a[static 0]) {}
-// NULL-INVALID: define void @test10(i32* nonnull %a)
-// NULL-VALID: define void @test10(i32* %a)
+// NULL-INVALID: define void @test10(i32* nonnull align 4 %a)
+// NULL-VALID: define void @test10(i32* align 4 %a)