Re: Inlining with gcc 4.x.x (Linux) ?

On 10/01/17 17:17, Waldek Hebisch wrote:
> peter wrote:
>>
>> I've built GPC 3.4.6 and 4.3.6 on Debian Stretch (64bit)
>> Running benchmarks on a Perlin Noise routine I get
>>
>> 3.4.6    5.6 secs
>> 4.3.6   11.0 secs   !!
>>
>> The difference seems to be mainly down to a failure of 4.3.6 to inline.
> <snip> 
>> Maybe I have broken my build of 4.3.6   Can anyone else conform the
>> status of inlining on linux x86 with 4.x.x compilers?  The simple
>> example from the info file
>>
>> program InlineDemo;
>>
>>     function Max (x, y: Integer): Integer; attribute (inline);
>>     begin
>>         if x > y then
>>            Max := x
>>         else
>>            Max := y
>>     end;
>>
>>  begin
>>         WriteLn (Max (42, 17), ' ', Max (-4, -2))
>>  end.
>>
>>
>> Also does not work for me with 4.3.6. It still produces a call instruction.
>> 	call	_p__M0_S0_Max
>>
>>
>> >From further tinkering around, I notice that 3.4.6 often inlines even
>> when not asked to do so, whereas 4.3.6 very rarely if ever inlines.
>> Neither compiler seems to obey the inline attribute!
>>
>> Anyone any thoughts on this?
>> Hoping of course that its an easy to fix typo type bug...
> 
> I will look into this.  One thing to remember is that gcc backend
> treat 'inline' only as a hint and apparently with newer backend
> gcc developers claim that compiler knows better then the programmer
> if inlining is good.  There is a way to request inlining regardless
> of compiler opinion about benefits, but this still may fail
> because compiler can inline only when certain problematic
> constructs are avoided.  For example nonlocal gotos disable
> inlining, even though for such routines inlining could be
> very profitable by converting nonlocal jumps to local ones.
> Also I am affraid that gpc currently does not implement needed
> directive.
> 
> If you use subranges or arrays you can try to disable range
> checking.  Gcc backend is quite good at removing redundant
> checks, but due to checking code routines may appear bigger
> and inliner may decide that inlining is too costly.
> 

Hi,

I had disabled range checks in the Noise routine, and disabling them in
the demo does'nt fix inlining. (Compiling with -O3)


I realise now that the patches only go up to 4.3.5 and also the bottom
of the assembler is
	.ident	"GCC: pkgversion_string ???experimental 20110215"
	
whereas for 3.4.6 I get
	.ident	"GCC: (GNU) 3.4.6"

Still, I pretty sure this was a problem with 4.1 (64bit) too.


Regards,
Peter







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