https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80155
bin cheng <amker at gcc dot gnu.org> changed:
What |Removed |Added
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CC| |amker at gcc dot gnu.org
--- Comment #35 from bin cheng <amker at gcc dot gnu.org> ---
(In reply to prathamesh3492 from comment #33)
> Created attachment 42341 [details]
> Test-case to reproduce regression with cortex-m7
>
> I have attached an artificial test-case that is fairly representative of the
> regression we are seeing in a benchmark. The test-case mimics a
> deterministic finite automaton. With code-hoisting there's an additional
> spill of r5 near beginning of the function.
>
...
>
> Without code-hoisting it is reusing r3 to store a + 1, while due to code
> hoisting it uses the extra register 'r2' to store the value of hoisted
> expression a + 1.
>
> Would it be a good idea to somehow "limit" the distance (in terms of number
> of basic blocks maybe?) between the definition of hoisted variable and it's
> furthest use during PRE ? If that exceeds a certain threshold then PRE
> should choose not to hoist that expression. The threshold could be a param
> that can be set by backends.
> Does this analysis look reasonable ?
It might be more accurate to calculate register pressure and use that to guide
code hoisting. I introduced register pressure hoisting for RTL under option
-fira-hoist-pressure, basically similar thing needs to be done here.
The proposed Tree-SSA register pressure patch set is still under review, but
please note it only does minimal now by only computing register pressure. To
make it useful in this case, it may need to be improved by
calculating/recording live range for statements (I did that in previous version
patch). We would also need interfaces updating live range information in line
with code motion.
I think this case is difficult also because it's hard to decide high register
pressure or not which could be the boundary case, and as we know, Tree-SSA
register pressure is in no way accurate.
Another difficulty is some kind of global information is needed. Given stupid
example:
a = ...
b = ...
loop
x = a + b;
...
y = a * b;
y = x + y;
store y;
end
The expressions need to be considered together in order to understand register
pressure change.
Thanks,
bin
>
> Thanks,
> Prathamesh
