1095740 - OdinMonkey: refine rules for 'const'

Status: RESOLVED FIXED

(Core :: JavaScript Engine: JIT, defect)

Description

[Luke Wagner [:luke]]

Attachment: const-tweaks

This patch tweaks the rules for const in asm.js:
 - It gives 'const' globals the exact type of the literal which is more precise than the 'int' type given to variables.
 - It allows const literals to be used without coercion in return statements (for symmetry with their use in var initializers).

Comment 1

[Benjamin Bouvier [:bbouvier] (inactive)]

Comment on attachment 8519198 [details] [diff] [review]
const-tweaks

Review of attachment 8519198 [details] [diff] [review]:
-----------------------------------------------------------------

Nice simplification!
Double check me here, but iiuc, there is no risk that this breaks demos (as before this patch, the validation rules were stricter; and some |0 or >>>0 were needed at places where this is not needed after this patch).

::: js/src/asmjs/AsmJSValidate.cpp
@@ +1567,5 @@
>      void initBufferArgumentName(PropertyName *n) { module_->initBufferArgumentName(n); }
>  
>      bool addGlobalVarInit(PropertyName *varName, const AsmJSNumLit &lit, bool isConst) {
>          uint32_t index;
> +        Type type = isConst ? Type::Of(lit) : VarType::Of(lit).toType();

Might be worth a comment that this distinction gives const integer variables their precise Type rather than just "int", what do you think?

::: js/src/jit-test/tests/asm.js/testControlFlow.js
@@ +18,5 @@
> +assertAsmTypeFail(USE_ASM + "const M = 42; function f() { return M } function g() { return +f() } return f");
> +assertEq(asmLink(asmCompile(USE_ASM + "const M = 42.1; function f() { return M } function g() { return +f() } return f"))(), 42.1);
> +assertAsmTypeFail(USE_ASM + "const M = 42.1; function f() { return M } function g() { return f()|0 } return f");
> +assertEq(asmLink(asmCompile('glob', USE_ASM + "var tof = glob.Math.fround; const M = tof(42); function f() { return M } function g() { return tof(f()) } return f"), this)(), 42);
> +assertAsmTypeFail('glob', USE_ASM + "var tof = glob.Math.fround; const M = tof(42); function f() { return M } function g() { return +f() } return f");

(really not important, but shouldn't these be in testCall instead?)

Comment 2

[Luke Wagner [:luke]]

(In reply to Benjamin Bouvier [:bbouvier] from comment #1)
> Double check me here, but iiuc, there is no risk that this breaks demos

That's right; the literal type is a subtype of the vartype.  Agreed with your other suggestions.

https://hg.mozilla.org/integration/mozilla-inbound/rev/c617a998c239

Comment 3

[Wes Kocher (:KWierso) (Not reading bugmail; email directly if needed)]

https://hg.mozilla.org/mozilla-central/rev/c617a998c239