RFR: 8320379: C2: Sort spilling/unspilling sequence for better ld/st merging into ldp/stp on AArch64

[Andrew Haley]

On Tue, 21 Nov 2023 10:25:04 GMT, Andrew Haley <aph at openjdk.org> wrote:

>> Macro-assembler on aarch64 can merge adjacent loads or stores into ldp/stp.[[1]](https://github.com/openjdk/jdk/blob/a95062b39a431b4937ab6e9e73de4d2b8ea1ac49/src/hotspot/cpu/aarch64/macroAssembler_aarch64.cpp#L2079)
>> 
>> For example, it can merge:
>> 
>> str     w20, [sp, #16]
>> str     w10, [sp, #20]
>> 
>> into
>> 
>> stp     w20, w10, [sp, #16]
>> 
>> 
>> But C2 may generate a sequence like:
>> 
>> str     x21, [sp, #8]
>> str     w20, [sp, #16]
>> str     x19, [sp, #24] <---
>> str     w10, [sp, #20] <--- Before sorting
>> str     x11, [sp, #40]
>> str     w13, [sp, #48]
>> str     x16, [sp, #56]
>> 
>> We can't do any merging for non-adjacent loads or stores.
>> 
>> The patch is to sort the spilling or unspilling sequence in the order of offset during instruction scheduling and bundling phase. After that, we can get a new sequence:
>> 
>> str     x21, [sp, #8]
>> str     w20, [sp, #16]
>> str     w10, [sp, #20] <---
>> str     x19, [sp, #24] <--- After sorting
>> str     x11, [sp, #40]
>> str     w13, [sp, #48]
>> str     x16, [sp, #56]
>> 
>> 
>> Then macro-assembler can do ld/st merging:
>> 
>> str     x21, [sp, #8]
>> stp     w20, w10, [sp, #16] <--- Merged
>> str     x19, [sp, #24]
>> str     x11, [sp, #40]
>> str     w13, [sp, #48]
>> str     x16, [sp, #56]
>> 
>> 
>> To justify the patch, we run `HelloWorld.java`
>> 
>> public class HelloWorld {
>>     public static void main(String [] args) {
>>         System.out.println("Hello World!");
>>     }
>> }
>> 
>> with `java -Xcomp -XX:-TieredCompilation HelloWorld`.
>> 
>> Before the patch, macro-assembler can do ld/st merging for 3688 times. After the patch, the number of ld/st merging increases to 3871 times, by ~5 %.
>> 
>> Tested tier1~3 on x86 and AArch64.
>
> This is a good idea, although the real-world gains are small. I'd wonder if this was worth doing for non-AArch64 ports, although even on others sorting the accesses into order might help.

> Hi @theRealAph , thanks a lot for your review! All comments have been resolved in the new commit.
> 
> > This is a good idea, although the real-world gains are small. I'd wonder if this was worth doing for non-AArch64 ports, although even on others sorting the accesses into order might help.
> 
> Yeah, that's also bothering me. I'm not sure if it benefits other ports. Do you think if we need convert the change to aarch64-only? Thanks.

#ifdefs are probably wrose, so I'd leave it as it is. We need another reviewer.

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PR Comment: https://git.openjdk.org/jdk/pull/16754#issuecomment-1824624806