2044243 - Use sign-extended tags for value tests

Status: RESOLVED FIXED

(Core :: JavaScript Engine: JIT, enhancement, P1)

Description

[André Bargull [:anba]]

Use sign-extended tags for value tests like we already do for ARM64.

function f(x) {
  return typeof x == "string";
}

Is compiled to:

srli      t4, t0, 47
lui       t5, 0x20
addiw     t5, t5, -10
xor       t0, t4, t5
seqz      t0, t0

This can be improved to:

srai      t0, t0, 47
xori      t0, t0, 0xfffffff6
seqz      t0, t0

And LIR=Unbox:Int32 can be improved from:

srli      t4, t0, 47
lui       t5, 0x20
addiw     t5, t5, -15
beq       t4, t5, 0
...

to:

srai      t4, t0, 47
li        t5, -15
beq       t4, t5, 0
...

Comment 1

[André Bargull [:anba]]

Attachment: Bug 2044243: Use sign-extended tags for value tests. r=jandem!

This is similar to ARM64 and requires fewer instructions to test value types.

For example:

function f(x) {
  return typeof x == "string";
}

Was compiled to:

srli      t4, t0, 47
lui       t5, 0x20
addiw     t5, t5, -10
xor       t0, t4, t5
seqz      t0, t0

And is now just:

srai      t0, t0, 47
xori      t0, t0, 0xfffffff6
seqz      t0, t0

Or:

function f(x) {
  return typeof x == "string";
}

Was compiled to:

srli      t4, t0, 47
lui       t5, 0x20
addiw     t5, t5, -15
sltu      t0, t5, t4
xori      t0, t0, 0x1

And is now compiled to just two instructions (with the changes from bug 2044242):

srai      t0, t0, 47
sltiu     t0, t0, -14

Fallible unbox to Int32 used:

srli      t4, t0, 47
lui       t5, 0x20
addiw     t5, t5, -15
beq       t4, t5, 0
...

And now requires one less instruction:

srai      t4, t0, 47
li        t5, -15
beq       t4, t5, 0
...

Comment 2

[Pulsebot]

Pushed by [e-mail]:
https://github.com/mozilla-firefox/firefox/commit/7708d0da72ad
https://hg.mozilla.org/integration/autoland/rev/1f2c5b30e657
Use sign-extended tags for value tests. r=jandem

Comment 3

[Cosmin Sabou [:CosminS]]

https://hg.mozilla.org/mozilla-central/rev/1f2c5b30e657