Re: [patch] MIPS/gcc: Revert removal of DImode shifts for 32-bit targets

[Nigel Stephens <nigel@mips.com>]

Maciej W. Rozycki wrote:

>On Fri, 23 Jul 2004, Ralf Baechle wrote:
>
>  
>
>>With a bit of hand waiving because haven't done benchmarks I guess Richard
>>might be right.  The subroutine calling overhead on modern processors is
>>rather low and smaller code means better cache hit rates ...
>>    
>>
>
> Well, I just worry the call may itself include at least the same number
>of instructions as the callee if inlined.  There would be no way for it to
>be faster.
>
> That may happen for a leaf function -- the call itself, plus $ra
>saving/restoration is already four instructions.  Now it's sufficient for
>two statics to be needed to preserve temporaries across such a call and
>the size of the caller is already the same.  With three statics, you lose
>even for a non-leaf function.  That's for a function containing a single
>call to such a shift -- if there are more, then you may win (but is it
>common?).
>
> So not only it may not be faster, but the resulting code may be bigger as
>well.  That said, the current GCC's implementation of these operations is
>not exactly optimal for current MIPS processors.  That's trivial to deal
>with in Linux, but would it be possible to pick a different implementation
>from libgcc based on the "-march=" setting, too?
>
> 
>

I second Maciej. My own recent experience when tuning the hell out of a 
software floating-point emulator was that efficient 64-bit shifts were 
really critical. I have a patch against gcc-3.4 which makes the 64-bit 
inline shifts somewhat smaller on ISAs which include the conditional 
move (movz/movn) instructions, but more importantly removes all branches 
from the inline code - which can be very expensive on long pipeline 
CPUs, since in this sort of code they tend to cause many branch 
mispredicts. Let me know if you want me to extract the patch - here's a 
table of the number of instructions generated by the original md pattern 
and the patched version:

		Instructions
		Old	New
ashldi3		12	9
ashrdi3		12	12
lshrdi3		12	9


If people really don't like the inline expansion, then maybe it could be 
enabled or disabled by a new -m option.

Nigel

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