Re: [PATCH][ PR rtl-optimization/79286] Drop may_trap_p exception to testing dominance in update_equiv_regs

[Bernd Edlinger <bernd dot edlinger at hotmail dot de>]

On 04/28/17 18:48, Jeff Law wrote:
> On 04/23/2017 05:54 AM, Bernd Edlinger wrote:
>> Hi Jeff,
>>
>>
>> this patch tries to fix the handling of pic_offset_rtx +
>> const(unspec(symbol_ref)+ const_int) in may_trap_p,
>> and restores the original state of affairs in update_equiv_regs.
>>
>>
>> What do you think is it OK to extract the symbol_ref out
>> of the unspec in this way, or is does it need a target hook?
>>
>>
>> Patch works at least for x86_64 and arm.
>> Is it OK for trunk?
>>
>>
>> Bernd.
>>
>>
>> patch-pr79286.diff
>>
>>
>> 2017-04-23  Bernd Edlinger<bernd.edlinger@hotmail.de>
>>
>>          rtl-optimizatoin/79286
>>          * 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.  Extract constant offset from pic_offset_table_rtx +
>>     const(unspec(symbol_ref)+int_val) and pic_offset_table_rtx +
>>     const(unspec(symbol_ref)), otherwise RTL may trap.
> ISTM that rtx_addr_can_trap_p_1 is fundamentally broken in that
> references via pic_offset_table_rtx can certainly trap.
>
> Whether or not a given reference traps is a function of the size of
> table (not known until link time) and the CONST_INT within the address
> expression.  So I'd be more inclined to remove the special casing of PIC
> addresses where entirely -- that seemed pretty risky during stage3, but
> now would be an appropriate time to tackle that.
>
> If we were to try and keep the special handling, you certainly can't
> depend on the form looking like (const(unspec(symbol_ref) + const_int).
>
> You could have high/lo_sums were and probably other forms too. We allow
> the backends to largely define what  PIC address looks like.
>

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
wrong.


Bernd.