On Mon, May 9, 2016 at 5:59 AM, Richard Biener <rguent...@suse.de> wrote:
>
> The following patch implements CSEing of "subreg" reads from memory
> like (from the testcase in the PR)
>
> union U { int i[16]; char c; };
>
> char foo(int i)
> {
> union U u;
> u.i[0] = i;
> return u.c;
> }
>
> CSEing u.c as (char)i and thus removing u during GIMPLE optimizations.
>
> The patch always goes via generating BIT_FIELD_REFs and letting them
> be simplified via the match-and-simplify machinery. This means it
> replaces handling of complex component and vector extracts we've been
> able to do before.
>
> I didn't restrict the kind of BIT_FIELD_REFs much apart from requiring
> byte-size accesses. I did inspect code generated on powerpc (big-endian)
> for the testcase though (also to verify any endianess issues) and didn't
> spot anything wrong (even for non-lowpart "subregs").
>
> Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.
>
> Richard.
>
> 2016-05-09 Richard Biener <rguent...@suse.de>
>
> PR tree-optimization/70497
> * tree-ssa-sccvn.c (vn_nary_build_or_lookup): New function
> split out from ...
> (visit_reference_op_load): ... here.
> (vn_reference_lookup_3): Use it to handle subreg-like accesses
> with simplified BIT_FIELD_REFs.
> * tree-ssa-pre.c (eliminate_insert): Handle inserting BIT_FIELD_REFs.
> * tree-complex.c (extract_component): Handle BIT_FIELD_REFs
> correctly.
>
> * gcc.dg/torture/20160404-1.c: New testcase.
> * gcc.dg/tree-ssa/ssa-fre-54.c: Likewise.
>
This caused:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71055
H.J.
