[PATCH 0/3] Add __builtin_load_no_speculate

[Richard Earnshaw (lists)]

On 05/01/18 11:35, Jakub Jelinek wrote:
> On Fri, Jan 05, 2018 at 10:59:21AM +0000, Richard Earnshaw (lists) wrote:
>>> But the condition could be just 'true' in the instruction encoding?  That is,
>>> why do you do both the jump-around and the csel?  Maybe I misunderstood
>>> the RTL you appear to generate.  I'd have expected an UNSPEC to avoid
>>> the compiler messing up things.
>>>
>>
>> That's why the intrinsic takes explicit bounds not a simple bool
>> expressing the conditions.  It prevents the optimizers from replacing
>> the condition by value range propagation.  That does restrict the
> 
> Preventing optimizers from replacing the condition can be done in many ways,
> IFN, UNSPEC, ...
> The question is if you really need it at the assembly level, or if you can
> just do an unconditional branch or branch conditional on say comparison of
> two constants, without any dynamic bounds.
> 
> 	Jakub
> 

The bounds /have/ to reflect the real speculation condition.  Otherwise
the CSEL becomes ineffective.

R.