[PATCH] D17950: Implement is_always_lock_free

[Ben Craig via cfe-commits]

bcraig added inline comments.

================
Comment at: lib/Frontend/InitPreprocessor.cpp:305
@@ +304,3 @@
+    if (TypeWidth == TypeAlign && (TypeWidth & (TypeWidth - 1)) == 0 &&
+        TypeWidth <= InlineWidth)
+      return Always;
----------------
jyknight wrote:
> jfb wrote:
> > bcraig wrote:
> > > On some targets (like Hexagon), 4-byte values are cheap to inline, but 1-byte values are not.  Clang is spotty about checking this, but TargetInfo::hasBuiltinAtomic seems like the right function to ask, if you have access to it.
> > You're commenting on:
> > ```
> >     if (TypeWidth == TypeAlign && (TypeWidth & (TypeWidth - 1)) == 0 &&
> >         TypeWidth <= InlineWidth)
> > ```
> > ?
> > 
> > Are you asking for a separate change, or for a change to my patch?
> That issue in clang's purview in any case; if hexagon wants to lower a 1-byte cmpxchg to a compiler runtime function, it should do so in its backend.
I am asking for a change to this patch.  Don't assume that all widths smaller than some value are inline-able and lock free, because that may not be the case.  A 4 byte cmpxchg could be lock free, while a 1 byte cmpxchg may not be lock free.


http://reviews.llvm.org/D17950