A strange bit of code in MacroAssembler::multiply_128_x_128_loop
vladimir.kozlov at oracle.com
Fri Apr 10 16:47:13 UTC 2015
On 4/10/15 9:30 AM, Vladimir Kozlov wrote:
> It restores counter after:
> subl(idx, 2);
> jcc(Assembler::negative, L_check_1);
> The value could be 1,2,3 before this point (idx & 3 before).
> After 'sub 2': -1, 0, 1.
> So we have to restore to positive values before subtracting 1.
And you are right, it is not needed. We get the same (correct) result from 'and idx,1' regardless executing 'add'.
> On 4/10/15 8:12 AM, Daniel D. Daugherty wrote:
>> Adding in the Compiler team since this is the MacroAssembler...
>> On 4/10/15 9:06 AM, Andrew Haley wrote:
>>> This is for x86:
>>> addl (idx, 0x2);
>>> andl (idx, 0x1);
>>> subl(idx, 1);
>>> jcc(Assembler::negative, L_post_third_loop_done);
>>> I'm trying to guess what the "addl (idx, 0x2)" instruction was supposed to do.
>>> I don't think it has any effect now.
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