RFR: 8287373: remove unnecessary paddings in generated code

[Vladimir Kozlov]

On Thu, 28 Apr 2022 14:46:57 GMT, Boris Ulasevich <bulasevich at openjdk.org> wrote:

> The goal is to remove unnecessary paddings in generated code. The alignment of the [Stub Code] section is determined by the same value as the alignment of the [Entry Point] section: the CodeEntryAlignment parameter with default values 64B on AARCH, and 32B on AMD.
> 
> Large entry alignment values are questionable for entry section. For example, Arm Neoverse N1 Software Optimization Guide recommends to align subroutines to 32B, while static compilers uses an even smaller value of 16B. However, with this change, I suggest to apply different (and smaller) values for [Constants] and [Stub Code] section alignments. This makes overall code 2% smaller on AARCH.
> 
> The correctness of the changes is checked by jtreg. Performance tested by Renaissance and SpecJBB benchmarkds on AARCH and AMD.
> 
> Example. Dummy method disassembly on AARCH, before vs after:
> 
> [Verified Entry Point]                          |  [Verified Entry Point]
>   78c63b80:   nop                               |    7437e480:   nop
>   78c63b84:   sub     x9, sp, #0x20, lsl #12    |    7437e484:   sub     x9, sp, #0x20, lsl #12
>   78c63b88:   str     xzr, [x9]                 |    7437e488:   str     xzr, [x9]
>   78c63b8c:   sub     sp, sp, #0x20             |    7437e48c:   sub     sp, sp, #0x20
>   78c63b90:   stp     x29, x30, [sp, #16]       |    7437e490:   stp     x29, x30, [sp, #16]
>   78c63b94:   orr     w1, wzr, #0x10            |    7437e494:   orr     w1, wzr, #0x10
>   78c63b98:   bl      78343e00                  |    7437e498:   bl      73a61980
>   78c63b9c:   .inst   0x00000000 ; undefined    |    7437e49c:   .inst   0x00000000 ; undefined
>   78c63ba0:   .inst   0x00000000 ; undefined    |
>   78c63ba4:   .inst   0x00000000 ; undefined    |
>   78c63ba8:   .inst   0x00000000 ; undefined    |
>   78c63bac:   .inst   0x00000000 ; undefined    |
>   78c63bb0:   .inst   0x00000000 ; undefined    |
>   78c63bb4:   .inst   0x00000000 ; undefined    |
>   78c63bb8:   .inst   0x00000000 ; undefined    |
>   78c63bbc:   .inst   0x00000000 ; undefined    |
> [Stub Code]                                     |  [Stub Code]
>   78c63bc0:   ldr     x8, 78c63bc8              |    7437e4a0:   ldr     x8, 7437e4a8
>   78c63bc4:   br      x8                        |    7437e4a4:   br      x8
>   78c63bc8:   .inst   0x78343e00 ; undefined    |    7437e4a8:   .inst   0x73a61980 ; undefined
>   78c63bcc:   .inst    ; undefined              |    7437e4ac:   .inst    ; undefined
> [Exception Handler]                             |  [Exception Handler]
>   78c63bd0:   b       783ee080                  |    7437e4b0:   b       73b0c100
> [Deopt Handler Code]                            |  [Deopt Handler Code]
>   78c63bd4:   adr     x30, 78c63bd4             |    7437e4b4:   adr     x30, 7437e4b4
>   78c63bd8:   b       78343ac0                  |    7437e4b8:   b       73a61620
>   78c63bdc:   .inst   0x00000000 ; undefined    |    7437e4bc:   .inst   0x00000000 ; undefined

Why you changed tested alignment to 8 for JVMCI tests?

src/hotspot/cpu/x86/sharedRuntime_x86_32.cpp line 2635:

> 2633:   // allocate space for the code
> 2634:   // setup code generation tools
> 2635:   CodeBuffer   buffer("handler_blob", 2048, 1024);

Why you need to double the size?

src/hotspot/share/jvmci/jvmciCodeInstaller.cpp line 667:

> 665:   int total_size = align_up(_constants_size, CodeSection::alignment(CodeBuffer::SECT_INSTS)) +
> 666:                    align_up(_code_size, CodeSection::alignment(CodeBuffer::SECT_STUBS)) +
> 667:                    stubs_size;

At first look it is messed up. I understand that you are trying to take into account space between sections. It assumed the order of sections. 
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PR: https://git.openjdk.java.net/jdk/pull/8453