[PATCH] D17950: Implement is_always_lock_free

[JF Bastien via cfe-commits]

jfb added inline comments.

================
Comment at: lib/Frontend/InitPreprocessor.cpp:305
@@ +304,3 @@
+    if (TypeWidth == TypeAlign && (TypeWidth & (TypeWidth - 1)) == 0 &&
+        TypeWidth <= InlineWidth)
+      return Always;
----------------
bcraig wrote:
> 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.
Are you asking me to change this line:
```
if (TypeWidth == TypeAlign && (TypeWidth & (TypeWidth - 1)) == 0 &&
    TypeWidth <= InlineWidth)
```
?

In what way?


Please be more specific.


http://reviews.llvm.org/D17950