1115754 - Misc assembler optimizations

Status: RESOLVED FIXED

(Core :: JavaScript Engine: JIT, enhancement)

Description

[Dan Gohman [:sunfish]]

Attachment: eax-encodings.patch

The following patches implement a variety of low-level x86 encoding optimizations.

First is a patch implementing optimized %eax encodings.

Comment 1

[Dan Gohman [:sunfish]]

Attachment: vex-shrink-moves.patch

This patch optimizes movaps/movapd/movdqa to use 2-byte VEX prefixes instead of 3-byte ones in more cases.

Comment 2

[Dan Gohman [:sunfish]]

Attachment: eax-mov-encodings.patch

This implements the %eax encoding optimizations for mov instructions.

Comment 3

[Dan Gohman [:sunfish]]

Attachment: jump-shrink.patch

This patch enables the use of 8-bit jump displacements for jumps where the destination is known (e.g. loop backedges).

Comment 4

[Jan de Mooij [:jandem]]

Comment on attachment 8541771 [details] [diff] [review]
vex-shrink-moves.patch

Review of attachment 8541771 [details] [diff] [review]:
-----------------------------------------------------------------

Clever.

Comment 5

[Jan de Mooij [:jandem]]

Comment on attachment 8541780 [details] [diff] [review]
jump-shrink.patch

Review of attachment 8541780 [details] [diff] [review]:
-----------------------------------------------------------------

Great, also nice cleanup.

::: js/src/jit/shared/BaseAssembler-x86-shared.h
@@ +2884,5 @@
> +        int32_t diff = dst.offset() - m_formatter.size();
> +        spew("jmp        .Llabel%d", dst.offset());
> +
> +        // The jump immediate is an offset from the end of the jump instruction.
> +        // An jump instrution is either 1 byte opcode and 1 byte offset, or 1

Nit: A jump instruction

@@ +2944,5 @@
> +        int32_t diff = dst.offset() - m_formatter.size();
> +        spew("j%s        .Llabel%d", nameCC(cond), dst.offset());
> +
> +        // The jump immediate is an offset from the end of the jump instruction.
> +        // A conditional jump instrution is either 1 byte opcode and 1 byte

Nit: instruction (typo)

Comment 6

[Dan Gohman [:sunfish]]

Nits fixed. Thanks!

https://hg.mozilla.org/integration/mozilla-inbound/rev/e5fbd8b624be
https://hg.mozilla.org/integration/mozilla-inbound/rev/39d80080db1c
https://hg.mozilla.org/integration/mozilla-inbound/rev/e6bd5d58f68a
https://hg.mozilla.org/integration/mozilla-inbound/rev/f6335334bbe8

Comment 7

[Nicolas B. Pierron [:nbp]]

Comment on attachment 8541780 [details] [diff] [review]
jump-shrink.patch

Review of attachment 8541780 [details] [diff] [review]:
-----------------------------------------------------------------

We should have a way to enforce an absolute address for jumpWithPatch, or a way to ensure that we will SEGV if we ever try to insert a larger address than what is acceptable by the jump size.
Note that loop backedges are patchable in order to handle interuption callbacks.

Comment 8

[Carsten Book [:Tomcat]]

https://hg.mozilla.org/mozilla-central/rev/f6335334bbe8
https://hg.mozilla.org/mozilla-central/rev/e6bd5d58f68a
https://hg.mozilla.org/mozilla-central/rev/39d80080db1c
https://hg.mozilla.org/mozilla-central/rev/e5fbd8b624be