1735481 - Spurious zero constant generated into register for addressing expression in 64-bit memories

Status: RESOLVED FIXED

(Core :: JavaScript: WebAssembly, enhancement, P3)

Description

[Lars T Hansen [:lth]]

Test case:

var mod = new WebAssembly.Module(wasmTextToBinary(
`(module
   (memory i64 1)
   (func (result i32)
     (i32.load (i64.const 16))))`));
wasmDis(mod)

Output with --wasm-compiler=ion --wasm-memory64:

00000010  55                        push %rbp
00000011  48 8b ec                  mov %rsp, %rbp
00000014  33 c0                     xor %eax, %eax
00000016  41 8b 44 07 10            movl 0x10(%r15,%rax,1), %eax
0000001B  5d                        pop %rbp
0000001C  c3                        ret

Instead of generating a zero with the xor and then using Base+Index addressing, we could have used a simple Address here (base+constant) and generated movl 0x10(%r15) as we do for 32-bit memories, and as we do for arm64 for that matter.

Comment 1

[Lars T Hansen [:lth]]

Attachment: Bug 1735481 - Memory64 - White-box tests for m64 bce. r?jseward

Currently a test case that fails because there's an interesting
wrinkle in code generation (or more likely a missing optimization
somewhere before code generation).

Depends on D127064

Comment 2

[Lars T Hansen [:lth]]

Attachment: Bug 1735481 - Memory64 - Make useRegisterOrZero work for i64 values. r?jseward

For addressing expressions we want to special-case zero. Now that
pointers can be 64-bit values, recognize also 64-bit zeroes.

Depends on D127064

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/320b7d0b5790
Memory64 - Make useRegisterOrZero work for i64 values.  r=jseward
https://hg.mozilla.org/integration/autoland/rev/d7d3cda7b6be
Memory64 - White-box tests for m64 bce. r=jseward

Comment 4

[Iulian Moraru]

https://hg.mozilla.org/mozilla-central/rev/320b7d0b5790
https://hg.mozilla.org/mozilla-central/rev/d7d3cda7b6be