Messages in this thread | | | Date | Sun, 17 Jul 2016 08:51:11 +0800 | From | Chen Gang <> | Subject | Re: [PATCH] mm: migrate: Use bool instead of int for the return value of PageMovable |
| |
On 7/13/16 15:53, Michal Hocko wrote: > On Wed 13-07-16 00:50:12, Chen Gang wrote: >> >> >> On 7/12/16 15:48, Michal Hocko wrote: >>> On Tue 12-07-16 03:47:42, Chen Gang wrote: >>> [...] >>>> In our case, the 2 output size are same, but under x86_64, the insns are >>>> different. After uses bool, it uses push/pop instead of branch, for me, >>>> it should be a little better for catching. >>> >>> The code generated for bool version looks much worse. Look at the fast >>> path. Gcc tries to reuse the retq from the fast path in the bool case >>> and so it has to push rbp and rbx on the stack. >>> >>> That being said, gcc doesn't seem to generate a better code for bool so >>> I do not think this is really worth it. >>> >> >> The code below also merge 3 statements into 1 return statement, although >> for me, it is a little more readable, it will generate a little bad code. >> That is the reason why the output looks a little bad. >> >> In our case, for gcc 6.0, using bool instead of int for bool function >> will get the same output under x86_64. > > If the output is same then there is no reason to change it. >
For the new version gcc, the output is same. But bool is a little more readable than int for the pure bool function.
But for the current widely used gcc version (I guess, gcc-4.8 is still widely used), bool will get a little better output than int for the pure bool function.
>> In our case, for gcc 4.8, using bool instead of int for bool function >> will get a little better output under x86_64. > > I had a different impression and the fast path code had more > instructions. But anyway, is there really a strong reason to change > those return values in the first place? Isn't that just a pointless code > churn? >
Excuse me, maybe, I do not quite understand your meanings, but I shall try to explain as far as I can understand (welcome additional detail explanation, e.g. "return values" means c code or assembly output code).
In the previous reply, I did not mention 3 things directly and clearly (about my 2 mistakes, and the comparation between gcc 6.0 and 4.8):
- Mistake 1: "Use one return statement instead of several statements" is not good, the modification may be a little more readable, but it may get a little bad output code by compiler.
- Mistake 2: I only notice there is more branches, but did not notice the real execution path (I guess, your "fast path" is about it).
- The optimization of upstream gcc 6.0 is better than redhat gcc 4.8: in this case, gcc 6.0 will:
generate the same better code for both bool and int for the pure bool function.
optimize the first checking branch (no prologue) -- gcc 4.8 need mark 'likely' for it.
skip the 'likely' optimization when "use 1 return statement instead of several statements" -- generation a little bad code too.
All together, for me:
- Only use bool instead of int for pure bool functions' return value will get a little better code
- I shall send patch v2, only change bool to int for all Page_XXX, and keep all the other things no touch.
Thanks. -- Chen Gang (陈刚)
Managing Natural Environments is the Duty of Human Beings.
| |