phiopt: Optimize x ? 1024 : 0 to (int) x << 10 [PR97690]
The following patch generalizes the x ? 1 : 0 -> (int) x optimization
to handle also left shifts by constant.
During x86_64-linux and i686-linux bootstraps + regtests it triggered
in 1514 unique non-LTO -m64 cases (sort -u on log mentioning
filename, function name and shift count) and 1866 -m32 cases.
Unfortunately, the patch regresses (before the tests have been adjusted):
+FAIL: gcc.dg/tree-ssa/ssa-ccp-11.c scan-tree-dump-times optimized "if " 0
+FAIL: gcc.dg/vect/bb-slp-pattern-2.c -flto -ffat-lto-objects scan-tree-dump-times slp1 "optimized: basic block" 1
+FAIL: gcc.dg/vect/bb-slp-pattern-2.c scan-tree-dump-times slp1 "optimized: basic block" 1
and in both cases it actually results in worse code.
> > We'd need some optimization that would go through all PHI edges and
> > compute if some use of the phi results don't actually compute a constant
> > across all the PHI edges - 1 & 0 and 0 & 1 is always 0.
> PRE should do this, IMHO only optimizing it at -O2 is fine.
> > Similarly, in the slp vectorization test there is:
> > a[0] = b[0] ? 1 : 7;
> So the option is to put : 7 in the 2, 4 an 8 case as well. The testcase
> wasn't added for any real-world case but is artificial I guess for
> COND_EXPR handling of invariants.
> But yeah, for things like SLP it means we eventually have to
> implement reverse transforms for all of this to make the lanes
> matching. But that's true anyway for things like x + 1 vs. x + 0
> or x / 3 vs. x / 2 or other simplifications we do.
2020-11-04 Jakub Jelinek <jakub@redhat.com>
PR tree-optimization/97690
* tree-ssa-phiopt.c (conditional_replacement): Also optimize
cond ? pow2p_cst : 0 as ((type) cond) << cst.
* gcc.dg/tree-ssa/phi-opt-22.c: New test.
* gcc.dg/tree-ssa/ssa-ccp-11.c: Use -O2 instead of -O1.
* gcc.dg/vect/bb-slp-pattern-2.c (foo): Use ? 2 : 7, ? 4 : 7 and
? 8 : 7 instead of ? 2 : 0, ? 4 : 0, ? 8 : 0.
git://gcc.gnu.org / gcc.git / commit