[PowerPC][AIX] Limit attribute aligned to 4096.

Limit the maximum alignment for attribute aligned to 4096 to match
the limit of the .align pseudo op in the system assembler.

Differential Revision: https://reviews.llvm.org/D107497

Author: mandlebug

clang/lib/Sema/SemaDeclAttr.cpp

@@ -4054,6 +4054,9 @@ void Sema::AddAlignedAttr(Decl *D, const AttributeCommonInfo &CI, Expr *E,
   unsigned MaximumAlignment = Sema::MaximumAlignment;
   if (Context.getTargetInfo().getTriple().isOSBinFormatCOFF())
     MaximumAlignment = std::min(MaximumAlignment, 8192u);
+  else if (Context.getTargetInfo().getTriple().isOSAIX())
+    MaximumAlignment = std::min(MaximumAlignment, 4096u);
+
   if (AlignVal > MaximumAlignment) {
     Diag(AttrLoc, diag::err_attribute_aligned_too_great)
         << MaximumAlignment << E->getSourceRange();

clang/test/Sema/aix-attr-aligned-limit.c

@@ -0,0 +1,4 @@
+// RUN: %clang_cc1 -triple powerpc-unknown-aix -fsyntax-only -verify %s
+// RUN: %clang_cc1 -triple powerpc64-unknown-aix -fsyntax-only -verify %s
+//
+int a __attribute__((aligned(8192))); // expected-error {{requested alignment must be 4096 bytes or smaller}}