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Subject: [Bug rtl-optimization/51821] [4.5/4.6/4.7 Regression] 64bit > 32bit conversion produces incorrect results with optimizations
Date: Thu, 12 Jan 2012 11:35:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=51821

--- Comment #13 from Uros Bizjak <ubizjak at gmail dot com> 2012-01-12 11:34:40 UTC ---
(In reply to comment #12)

> > Here we need to analyse the insn patterns for ALL sets and clobbers, not only
> > track live registers through the insn stream.
> 
> I'm not sure I understand.  If the peephole matches, then the insn pattern is
> present in the insn stream with instantiated registers, so it's sufficient to
> scan the insn stream.  AFAICS the code doesn't see that edx is live after the
> instruction because DF reports that only eax is; of course it's eax in DImode
> so edx is "implicitly" live, but it's nevertheless live.

Please look into "peep2_insn_data[from].live_before" array and
"peep2_insn_data[from+1].live_before". You will see that the difference is only
in ax register. This array is set with df_simulate_one_insn_forwards, and this
particular function indeed clears set hard registers when REG_UNUSED note is
found (this is OK for live analysis).

The referred loop processes

(insn 7 19 16 2 (parallel [
            (set (reg:DI 0 ax [65])
                (ashift:DI (const_int -1 [0xffffffffffffffff])
                    (reg:QI 2 cx [orig:63 shift_size ] [63])))
            (clobber (reg:CC 17 flags))
        ]) pr51821.c:8 489 {*ashldi3_doubleword}
     (expr_list:REG_DEAD (reg:QI 2 cx [orig:63 shift_size ] [63])
        (expr_list:REG_UNUSED (reg:CC 17 flags)
            (expr_list:REG_UNUSED (reg:SI 1 dx)
                (expr_list:REG_EQUAL (ashift:DI (const_int -1
[0xffffffffffffffff])
                        (subreg:QI (reg/v:SI 2 cx [orig:63 shift_size ] [63])
0))
                    (nil))))))

and since the difference between live registers before/after the insn is only
ax register, the loop claims others (including dx reg) as "available". The
calculation, which register is _NOT_CLOBBERED_ by the pattern from "live"
information is thus not enough, we have to look into the pattern and mark all
hard registers that are set or clobbered as _NOT_AVAILABLE_ in the register
set.