Re: [PATCH][PR90106] Builtin call transformation changes in cdce pass

[JunMa <JunMa at linux dot alibaba dot com>]

在 2019/5/10 上午4:00, Jeff Law 写道:
> On 5/8/19 8:25 PM, JunMa wrote:
> > 在 2019/5/9 上午9:20, Bin.Cheng 写道:
> > > On Thu, May 9, 2019 at 5:31 AM Jeff Law <law@redhat.com> wrote:
> > > > On 5/8/19 6:28 AM, Richard Biener wrote:
> > > > > On Wed, May 8, 2019 at 12:09 PM JunMa <JunMa@linux.alibaba.com> wrote:
> > > > > > Hi
> > > > > >
> > > > > > As PR90106 (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90106),
> > > > > > when gcc meets builtin function call like:
> > > > > >
> > > > > >      y = sqrt (x);
> > > > > >
> > > > > > The cdce pass tries to transform the call into an internal function
> > > > > > call and conditionally executes call with a simple range check on the
> > > > > > arguments which can detect most cases and the errno does not need
> > > > > > to be set. It looks like:
> > > > > >
> > > > > >      y = IFN_SQRT (x);
> > > > > >      if (__builtin_isless (x, 0))
> > > > > >        sqrt (x);
> > > > > >
> > > > > > However, If the call is in tail position, for example:
> > > > > >
> > > > > >      y =  sqrt (x);
> > > > > >      return y;
> > > > > >
> > > > > > will become:
> > > > > >
> > > > > >      y = IFN_SQRT (x);
> > > > > >      if (__builtin_isless (x, 0))
> > > > > >        sqrt (x);
> > > > > >      return y;
> > > > > >
> > > > > > This transformation breaks tailcall pattern, and prevents
> > > > > > later tailcall optimizations.
> > > > > >
> > > > > > So This patch transform builtin call with return value into
> > > > > > if-then-else part, which looks like:
> > > > > >
> > > > > >      y =  sqrt (x);
> > > > > >       ==>
> > > > > >      if (__builtin_isless (x, 0))
> > > > > >        y = sqrt (x);
> > > > > >      else
> > > > > >        y = IFN_SQRT (x);
> > > > > >
> > > > > > BTW, y = sqrt (x) can also transform like:
> > > > > >
> > > > > >      y = IFN_SQRT (x);
> > > > > >      if (__builtin_isless (x, 0))
> > > > > >        y = sqrt (x);
> > > > > >
> > > > > > We don‘t choose this pattern because it emits worse assemble
> > > > > > code(more move instruction and use more registers) in x86_64.
> > > > > >
> > > > > > Bootstrapped/regtested on x86_64-linux, ok for trunk?
> > > > > OK.
> > > > JunMa -- do you have a copyright assignment on file and write access to
> > > > the repository?  If not we should take care of that before proceeding
> > > > further.
> > > Hi Jeff,
> > > Thanks very much for helping.
> > > Yes, he is under Alibaba's copyright assignment.  What else should we
> > > do other than noticing in this mailing list message?
> > > BTW, I think JunMa needs to apply write-access though.
> > Yes, I do not have write access, FYI.
> OK.  So in my response to Bin I gave you a link to the form to fill out
> to get write access.  List me as your sponsor.  Once that's all taken
> care of and working, go ahead and commit this change to the trunk.
Thanks!
JunMa
> Jeff