On Thu, Sep 25, 2014 at 7:44 PM, Uros Bizjak <ubiz...@gmail.com> wrote:
>>> The failure was caused by barrier detection code, which failed to
>>> detect barrier after call insn was to be split when
>>> NOTE_CALL_ARG_LOCATION was present. This problem caused
>>> -fcompare-debug failure.
>>>
>>> Digging a bit deeped, and as hinted in the PR, the handling of
>>> barriers in try_split seems to be broken. The code is emitting extra
>>> barrier for non-debug compiles, but it "forgots" to remove the
>>> existing one, leading to duplicated barriers. The barrier is not
>>> detected at all for debug build.
>>>
>>> I have removed special handling of barriers here (also, the comment in
>>> removed code was not helpful at all), and this solved -fcompare-debug
>>> failure.
>>>
>>> The patch was also bootstrapped and regression tested on
>>> x86_64-linux-gnu {,-m32} which in -m32 mode splits x87 FP jump insns,
>>> and there were no regressions. However, I am not too familiar with
>>> rtl-optimization part and I am not confident that this code surgery is
>>> fully correct, so this is the reason for RFC status of the patch.
>>>
>>> 2014-09-24 Uros Bizjak <ubiz...@gmail.com>
>>>
>>> PR rtl-optimization/63348
>>> * emit-rtl.c (try_split): Do not emit extra barrier.
>>
>> Good grief, the code you're removing pre-dates any version control we have.
>> ie, it's in the first revision of emit-rtl.c from 1992. Egad.
>>
>> It's going to be a hell of a time figuring out why that code exists in the
>> first place. I don't like removing code if we don't know why the code
>> exists... Any reason you picked that route rather than looking forward
>> through the NOTEs to see if they're followed by a suitable BARRIER?
>
> I have tried with alternative patch that just skipped the NOTE:
>
> --cut here--
> Index: emit-rtl.c
> ===================================================================
> --- emit-rtl.c (revision 215606)
> +++ emit-rtl.c (working copy)
> @@ -3622,6 +3622,10 @@ try_split (rtx pat, rtx uncast_trial, int last)
> int njumps = 0;
> rtx call_insn = NULL_RTX;
>
> + if (after && NOTE_P (after)
> + && NOTE_KIND (after) == NOTE_INSN_CALL_ARG_LOCATION)
> + after = NEXT_INSN (after);
> +
> /* We're not good at redistributing frame information. */
> if (RTX_FRAME_RELATED_P (trial))
> return trial;
> --cut here--
>
> and resulted in:
>
> (call_insn 184 190 185 (parallel [
> (call (mem:SI (reg:SI 25 $25 [217]) [0 S4 A32])
> (const_int 16 [0x10]))
> (clobber (reg:SI 31 $31))
> (clobber (reg:SI 28 $28))
> ]) pr43670.c:29 595 {call_split}
> (expr_list:REG_NORETURN (const_int 0 [0])
> (nil))
> (expr_list (use (reg:SI 79 $fakec))
> (expr_list (use (reg:SI 28 $28))
> (nil))))
> (barrier 185 184 175)
> (note 175 185 130 (expr_list:REG_DEP_TRUE (concat:SI (pc)
> (unspec:SI [
> (reg:SI 28 $28)
> (const:SI (unspec:SI [
> (symbol_ref:SI ("abort") [flags 0x41]
> <function_decl 0x7fe1e3af2e58 abort>)
> ] 227))
> (reg:SI 79 $fakec)
> ] UNSPEC_LOAD_CALL))
> (nil)) NOTE_INSN_CALL_ARG_LOCATION)
> (barrier 130 175 174)
>
> I have noticed that the barrier is always there, since without -g, we have:
>
> (call_insn 76 82 77 (parallel [
> (call (mem:SI (reg:SI 25 $25 [217]) [0 S4 A32])
> (const_int 16 [0x10]))
> (clobber (reg:SI 31 $31))
> (clobber (reg:SI 28 $28))
> ]) pr43670.c:29 595 {call_split}
> (expr_list:REG_NORETURN (const_int 0 [0])
> (nil))
> (expr_list (use (reg:SI 79 $fakec))
> (expr_list (use (reg:SI 28 $28))
> (nil))))
> (barrier 77 76 37)
> (barrier 37 77 40)
>
> and considering the fact that the code didn't process barriers
> correctly with -g, I simply removed the emission. The - probably
> stalled - comment was not helpful at all.
FYI, unpatched gcc created (-g):
(call_insn 184 189 175 (parallel [
(call (mem:SI (reg:SI 25 $25 [217]) [0 S4 A32])
(const_int 16 [0x10]))
(clobber (reg:SI 31 $31))
(clobber (reg:SI 28 $28))
]) pr43670.c:29 595 {call_split}
(expr_list:REG_NORETURN (const_int 0 [0])
(nil))
(expr_list (use (reg:SI 79 $fakec))
(expr_list (use (reg:SI 28 $28))
(nil))))
(note 175 184 130 (expr_list:REG_DEP_TRUE (concat:SI (pc)
(unspec:SI [
(reg:SI 28 $28)
(const:SI (unspec:SI [
(symbol_ref:SI ("abort") [flags 0x41]
<function_decl 0x7fc774a8ee58 abort>)
] 227))
(reg:SI 79 $fakec)
] UNSPEC_LOAD_CALL))
(nil)) NOTE_INSN_CALL_ARG_LOCATION)
(barrier 130 175 174)
so, it didn't emit barrier at all.
Uros.
