Is it OK for rtx_addr_can_trap_p_1 to attempt to compute the frame layout? (was Re: [PATCH] Skip re-computing the mips frame info after reload completed)

[Bernd Schmidt]

On 01/29/2016 08:42 PM, Bernd Edlinger wrote:
> On 29.01.2016 16:47 Bernd Schmidt wrote:
>>
>> Yes. What is the problem with that? If we have (plus sfp const_int) at
>> any point before reload, we can check whether that offset is inside
>> frame_size. If it isn't or if the offset isn't known, it could trap.
>>
>>
>
> Usually we have "if (x==1234) { read MEM[FP+x]; }", so wo don't know,
> and then after reload: "if (x==1234) { read MEM[SP+x+sp_fp_offset]; }"
> but wait, in the if statement we know, that x==1234, so everything
> turns in one magic constant, and we have a totally new constant offset
> from the SP register "if (x==1234) { read MEM[SP+1234+sp_fp_offset]; }".
> Now if rtx_addr_can_trap_p(MEM[SP+1234+sp_fp_offset]) says it cannot
> trap we think we do not need the if at all => BANG.

What are you trying to say here? As far as I can tell this isn't a 
problem with my proposed solution (set MEM_NOTRAP_P for valid SFP+x 
offsets).


Bernd