[Clang] Propagate guaranteed alignment for malloc and others

LLVM should be smarter about *known* malloc's alignment and this knowledge may enable other optimizations.

Originally started as LLVM patch - https://reviews.llvm.org/D100862 but this logic should be really in Clang.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D100879

1 files changed, 18 insertions, 0 deletions

diff --git a/clang/lib/CodeGen/CGCall.cpp b/clang/lib/CodeGen/CGCall.cpp
index 366c83e..6c77f18 100644
--- a/clang/lib/CodeGen/CGCall.cpp
+++ b/clang/lib/CodeGen/CGCall.cpp
@@ -2048,6 +2048,24 @@ void CodeGenModule::ConstructAttributeList(
       // allows it to work on indirect virtual function calls.
       if (AttrOnCallSite && TargetDecl->hasAttr<NoMergeAttr>())
         FuncAttrs.addAttribute(llvm::Attribute::NoMerge);
+
+      // Add known guaranteed alignment for allocation functions.
+      if (unsigned BuiltinID = Fn->getBuiltinID()) {
+        switch (BuiltinID) {
+        case Builtin::BIaligned_alloc:
+        case Builtin::BIcalloc:
+        case Builtin::BImalloc:
+        case Builtin::BImemalign:
+        case Builtin::BIrealloc:
+        case Builtin::BIstrdup:
+        case Builtin::BIstrndup:
+          RetAttrs.addAlignmentAttr(Context.getTargetInfo().getNewAlign() /
+                                    Context.getTargetInfo().getCharWidth());
+          break;
+        default:
+          break;
+        }
+      }
     }
 
     // 'const', 'pure' and 'noalias' attributed functions are also nounwind.