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Re: [PATCH][ PR rtl-optimization/79286] Drop may_trap_p exception to testing dominance in update_equiv_regs
- From: Jeff Law <law at redhat dot com>
- To: Bernd Edlinger <bernd dot edlinger at hotmail dot de>, "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>
- Date: Thu, 22 Jun 2017 22:35:33 -0600
- Subject: Re: [PATCH][ PR rtl-optimization/79286] Drop may_trap_p exception to testing dominance in update_equiv_regs
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On 04/29/2017 01:06 AM, Bernd Edlinger wrote:
> On 04/28/17 20:46, Jeff Law wrote:
>> On 04/28/2017 11:27 AM, Bernd Edlinger wrote:
>>> Yes I agree, that is probably not worth it. So I could try to remove
>>> the special handling of PIC+const and see what happens.
>>> However the SYMBOL_REF_FUNCTION_P is another story, that part I would
>>> like to keep: It happens quite often, already w/o -fpic that call
>>> statements are using SYMBOL_REFs to ordinary (not weak) function
>>> symbols, and may_trap returns 1 for these call statements wihch is IMHO
>> Hmm, thinking more about this, wasn't the original case a PIC referrence
>> for something like &x[BIGNUM].
>> Perhaps we could consider a PIC reference without other arithmetic as
>> safe. That would likely pick up the SYMBOL_REF_FUNCTION_P case you want
>> as well good deal many more PIC references as non-trapping.
> Yes, I like this idea.
> I tried to compile openssl with -m32 -fpic as an example, and counted
> how often the mem[pic+const] is hit: that was 2353 times, all kind of
> object refs.
> Then I tried your idea, and only 54 unhandled pic refs remained, all of
> them looking like this:
> (plus:SI (reg:SI 107)
> (const:SI (plus:SI (unspec:SI [
> (symbol_ref:SI ("bf_init") [flags 0x2] <var_decl
> 0x2ac00f7bac60 bf_init>)
> ] UNSPEC_GOTOFF)
> (const_int 4164 [0x1044]))))
> I believe that is a negligible fall out from such a big code base.
> Although the pic references do no longer reach the
> SYMBOL_REF_FUNCTION_P in this version of the patch, I still see
> that happening without -fpic option, so I left it as is.
> Attached is the new version of my patch.
> Bootstrapped and reg-tested on x86_64-pc-linux-gnu.
> Is it OK for trunk?
> 2017-04-29 Bernd Edlinger <firstname.lastname@example.org>
> * ira.c (update_equiv_regs): Revert to using may_tap_p again.
> * rtlanal.c (rtx_addr_can_trap_p_1): SYMBOL_REF_FUNCTION_P can never
> trap. PIC register plus a const unspec without offset can never trap.
OK. Sorry for the delay. I've been swamped.