Re: [PATCH 4/4] rs6000: build constant via li/lis;rldic

[guojiufu <guojiufu@linux.ibm.com>]

On 2023-06-13 17:18, Jiufu Guo via Gcc-patches wrote:
> Hi David,
> 
> Thanks for your valuable comments!
> 
> David Edelsohn <dje.gcc@gmail.com> writes:
>> 
...
>> Do you have any measurement of how expensive it is to test all of 
>> these additional methods to generate a constant?  How much does this 
>> affect the
>> compile time?
> 
> Yeap, Thanks for this very good question!
> This patch is mostly using bitwise operations and if-conditions,
> it would be expected not expensive.
> 
> Testcases were checked.  For example:
> A case with ~1000 constants: most of them hit this feature.
> With this feature, the compiling time is slightly faster.
> 
> 0m1.985s(without patch) vs. 0m1.874s(with patch)
> (note:D rs6000_emit_set_long_const does not occur in hot perf
> functions.  So, the tricky time saving would not directly cause
> by this feature.)
> 
> A case with ~1000 constants:(most are not hit by this feature)
> 0m2.493s(without patch) vs. 0m2.558s(with patch).

Typo, this should be:
0m2.493s(with patch) vs. 0m2.558s(without patch).

It is also faster with the patch :)

BR,
Jeff (Jiufu Guo)

> 
> For runtime, actually, with the patch, it seems there is no visible
> improvement in SPEC2017.  While I still feel this patch is
> doing the right thing: use fewer instructions to build the constant.
> 
> BR,
> Jeff (Jiufu Guo)
> 
>> 
>> Thanks, David
>>