RFR: 8326541: [AArch64] ZGC C2 load barrier stub considers the length of live registers when spilling registers

[Stuart Monteith]

On Fri, 23 Feb 2024 08:11:24 GMT, Joshua Zhu <jzhu at openjdk.org> wrote:

> Currently ZGC C2 load barrier stub saves the whole live register regardless of what size of register is live on aarch64.
> Considering the size of SVE register is an implementation-defined multiple of 128 bits, up to 2048 bits,
> even the use of a floating point may cause the maximum 2048 bits stack occupied.
> Hence I would like to introduce this change on aarch64: take the length of live registers into consideration in ZGC C2 load barrier stub.
> 
> In a floating point case on 2048 bits SVE machine, the following ZLoadBarrierStubC2 
> 
> 
>   ......
>   0x0000ffff684cfad8:   stp     x15, x18, [sp, #80]
>   0x0000ffff684cfadc:   sub     sp, sp, #0x100
>   0x0000ffff684cfae0:   str     z16, [sp]
>   0x0000ffff684cfae4:   add     x1, x13, #0x10
>   0x0000ffff684cfae8:   mov     x0, x16
>  ;; 0xFFFF803F5414
>   0x0000ffff684cfaec:   mov     x8, #0x5414                     // #21524
>   0x0000ffff684cfaf0:   movk    x8, #0x803f, lsl #16
>   0x0000ffff684cfaf4:   movk    x8, #0xffff, lsl #32
>   0x0000ffff684cfaf8:   blr     x8
>   0x0000ffff684cfafc:   mov     x16, x0
>   0x0000ffff684cfb00:   ldr     z16, [sp]
>   0x0000ffff684cfb04:   add     sp, sp, #0x100
>   0x0000ffff684cfb08:   ptrue   p7.b
>   0x0000ffff684cfb0c:   ldp     x4, x5, [sp, #16]
>   ......
> 
> 
> could be optimized into:
> 
> 
>   ......  
>   0x0000ffff684cfa50:   stp     x15, x18, [sp, #80]
>   0x0000ffff684cfa54:   str     d16, [sp, #-16]!                   // extra 8 bytes to align 16 bytes in push_fp()
>   0x0000ffff684cfa58:   add     x1, x13, #0x10
>   0x0000ffff684cfa5c:   mov     x0, x16
>  ;; 0xFFFF7FA942A8
>   0x0000ffff684cfa60:   mov     x8, #0x42a8                     // #17064
>   0x0000ffff684cfa64:   movk    x8, #0x7fa9, lsl #16
>   0x0000ffff684cfa68:   movk    x8, #0xffff, lsl #32
>   0x0000ffff684cfa6c:   blr     x8
>   0x0000ffff684cfa70:   mov     x16, x0
>   0x0000ffff684cfa74:   ldr     d16, [sp], #16
>   0x0000ffff684cfa78:   ptrue   p7.b
>   0x0000ffff684cfa7c:   ldp     x4, x5, [sp, #16]
>   ......
> 
> 
> Besides the above benefit, when we know what size of register is live,
> we could remove the unnecessary caller save in ZGC C2 load barrier stub when we meet C-ABI SOE fp registers.
> 
> Passed jtreg with option "-XX:+UseZGC -XX:+ZGenerational" with no failures introduced.

That is a welcome change - in light of possibly very large registers, we don't want to save more than is necessary.
It looks OK, but have you run a specific test that demonstrates it working?

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PR Comment: https://git.openjdk.org/jdk/pull/17977#issuecomment-1969148590