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Bad and/or stupid code for DImode compares with gcc 4.6.1
- From: Alan Lehotsky <alehotsky at me dot com>
- To: gcc at gcc dot gnu dot org
- Date: Wed, 31 Oct 2012 13:51:52 -0400
- Subject: Bad and/or stupid code for DImode compares with gcc 4.6.1
- Reply-to: apl at alum dot mit dot edu
I'm looking at code generated for a new port of gcc using 4.6.1 and failing execute/950607-2.c with -O0 only
The target chip has only 32 bit instructions, so it's using do_jump_by_parts_<relop>_rtx() to expand the compare.
I've set up my .md to use the CCmode.
I see one case that seems really stupid, and one that's just wrong. I'm thinking that either I have something really flawed with my port's handing of DImode or that there was a bug in 4.6.1. The port is only failing about 2100 dejagnu tests (passing 64000+) and a good chunk of the failures are due to the ridiculously small data-memory size of the chip.
long long int x;
if ( x < 0 ) return 0 else return 2;
I see code that compares MSBs and branches on < (less than) as expected. But then it goes and checks the MSBs for != , and finally it checks the LSBS and emits a conditional branch to the ELSE, followed by an unconditional branch to the ELSE, so that I end up with code that looks like
cmpi $r2,0 <=== totally redundant for "x < 0" comparisons
.L5 : movi $r1, 0
.L2 : movi $r1, 2
This is a simplification of 950607-2.c, which fails at -O0, but passes at higher optimization levels (go figure...)