RFR: JDK-8274795: AArch64: avoid spilling and restoring r18 in macro assembler

[Bernhard Urban-Forster]

On Tue, 5 Oct 2021 20:33:31 GMT, Bernhard Urban-Forster <burban at openjdk.org> wrote:

> r18 should not be used as it is reserved as platform register. Linux is fine with userspace using it, but Windows and also recently macOS (https://github.com/openjdk/jdk11u-dev/pull/301#issuecomment-911998917 ) are actually using it on the kernel side.
> 
> The macro assembler uses the bit pattern `0x7fff_ffff` (== `r0-r30`) to specify which registers to spill; fortunately this helper is only used here:
> 
> https://github.com/openjdk/jdk/blob/c05dc268acaf87236f30cf700ea3ac778e3b20e5/src/hotspot/cpu/aarch64/templateInterpreterGenerator_aarch64.cpp#L1400-L1404
> 
> I haven't seen causing this particular instance any issues in practice _yet_, presumably because it looks hard to align the stars in order to trigger a problem (between `stp` and `ldp` of `r18` a transition to kernel space must happen *and* the kernel needs to do something with `r18`). But jdk11u-dev has more usages of the `::pusha`/`::popa` macro and that causes troubles as explained in the link above.
> 
> Output of `-XX:+PrintInterpreter` before this change:
> 
> ----------------------------------------------------------------------
> method entry point (kind = native)  [0x0000000138809b00, 0x000000013880a280]  1920 bytes
> --------------------------------------------------------------------------------
>   0x0000000138809b00:   ldr x2, [x12, #16]
>   0x0000000138809b04:   ldrh    w2, [x2, #44]
>   0x0000000138809b08:   add x24, x20, x2, uxtx #3
>   0x0000000138809b0c:   sub x24, x24, #0x8
> [...]
>   0x0000000138809fa4:   stp x16, x17, [sp, #128]
>   0x0000000138809fa8:   stp x18, x19, [sp, #144]
>   0x0000000138809fac:   stp x20, x21, [sp, #160]
> [...]
>   0x0000000138809fc0:   stp x30, xzr, [sp, #240]
>   0x0000000138809fc4:   mov x0, x28
>  ;; 0x10864ACCC
>   0x0000000138809fc8:   mov x9, #0xaccc                 // #44236
>   0x0000000138809fcc:   movk    x9, #0x864, lsl #16
>   0x0000000138809fd0:   movk    x9, #0x1, lsl #32
>   0x0000000138809fd4:   blr x9
>   0x0000000138809fd8:   ldp x2, x3, [sp, #16]
> [...]
>   0x0000000138809ff4:   ldp x16, x17, [sp, #128]
>   0x0000000138809ff8:   ldp x18, x19, [sp, #144]
>   0x0000000138809ffc:   ldp x20, x21, [sp, #160]
> 
> 
> After:
> 
> ----------------------------------------------------------------------
> method entry point (kind = native)  [0x0000000108e4db00, 0x0000000108e4e280]  1920 bytes
> 
> --------------------------------------------------------------------------------
>   0x0000000108e4db00:   ldr x2, [x12, #16]
>   0x0000000108e4db04:   ldrh    w2, [x2, #44]
>   0x0000000108e4db08:   add x24, x20, x2, uxtx #3
>   0x0000000108e4db0c:   sub x24, x24, #0x8
> [...]
>   0x0000000108e4dfa4:   stp x16, x17, [sp, #128]
>   0x0000000108e4dfa8:   stp x19, x20, [sp, #144]
>   0x0000000108e4dfac:   stp x21, x22, [sp, #160]
> [...]
>   0x0000000108e4dfbc:   stp x29, x30, [sp, #224]
>   0x0000000108e4dfc0:   mov x0, x28
>  ;; 0x107E4A06C
>   0x0000000108e4dfc4:   mov x9, #0xa06c                 // #41068
>   0x0000000108e4dfc8:   movk    x9, #0x7e4, lsl #16
>   0x0000000108e4dfcc:   movk    x9, #0x1, lsl #32
>   0x0000000108e4dfd0:   blr x9
>   0x0000000108e4dfd4:   ldp x2, x3, [sp, #16]
> [...]
>   0x0000000108e4dff0:   ldp x16, x17, [sp, #128]
>   0x0000000108e4dff4:   ldp x19, x20, [sp, #144]
>   0x0000000108e4dff8:   ldp x21, x22, [sp, #160]
> [...]

Thanks for the review Andrew.

Restore looks like this now:
    
    
 ```
      0x0000000106e4dfcc:   movk    x9, #0x5e4, lsl #16
      0x0000000106e4dfd0:   movk    x9, #0x1, lsl #32
      0x0000000106e4dfd4:   blr x9
      0x0000000106e4dfd8:   ldp x2, x3, [sp, #16]
      0x0000000106e4dfdc:   ldp x4, x5, [sp, #32]
      0x0000000106e4dfe0:   ldp x6, x7, [sp, #48]
      0x0000000106e4dfe4:   ldp x8, x9, [sp, #64]
      0x0000000106e4dfe8:   ldp x10, x11, [sp, #80]
      0x0000000106e4dfec:   ldp x12, x13, [sp, #96]
      0x0000000106e4dff0:   ldp x14, x15, [sp, #112]
      0x0000000106e4dff4:   ldp x16, x17, [sp, #128]
      0x0000000106e4dff8:   ldp x0, x1, [sp], #144
      0x0000000106e4dffc:   ldp xzr, x19, [sp], #16
      0x0000000106e4e000:   ldp x22, x23, [sp, #16]
      0x0000000106e4e004:   ldp x24, x25, [sp, #32]
      0x0000000106e4e008:   ldp x26, x27, [sp, #48]
      0x0000000106e4e00c:   ldp x28, x29, [sp, #64]
      0x0000000106e4e010:   ldp x30, xzr, [sp, #80]
      0x0000000106e4e014:   ldp x20, x21, [sp], #96
      0x0000000106e4e018:   ldur    x12, [x29, #-24]
      0x0000000106e4e01c:   ldr x22, [x12, #16]
      0x0000000106e4e020:   add x22, x22, #0x30
      0x0000000106e4e024:   ldr x8, [x28, #8]

-------------

PR: https://git.openjdk.java.net/jdk/pull/5828