1506502 - JS::Value Private API comments doesn't match the implementation

Status: RESOLVED DUPLICATE of bug 1505902

(Core :: JavaScript Engine, defect, P3)

Description

[Tooru Fujisawa [:arai]]

https://searchfox.org/mozilla-central/rev/06d5d5ae4396be85f26e8548323ee6c12e7bce4e/js/public/Value.h#858-865
>     /*
>      * Private API
>      *
>      * Private setters/getters allow the caller to read/write arbitrary types
>      * that fit in the 64-bit payload. It is the caller's responsibility, after
>      * storing to a value with setPrivateX to read only using getPrivateX.
>      * Privates values are given a type which ensures they are not marked.
>      */
> 
>     void setPrivate(void* ptr) {
>         MOZ_ASSERT((uintptr_t(ptr) & 1) == 0);
> #if defined(JS_NUNBOX32)
>         s_.tag_ = JSValueTag(0);
>         s_.payload_.ptr_ = ptr;
> #elif defined(JS_PUNBOX64)
>         asBits_ = uintptr_t(ptr) >> 1;
> #endif
>         MOZ_ASSERT(isDouble());
>     }
> 
>     void* toPrivate() const {
>         MOZ_ASSERT(isDouble());
> #if defined(JS_NUNBOX32)
>         return s_.payload_.ptr_;
> #elif defined(JS_PUNBOX64)
>         MOZ_ASSERT((asBits_ & 0x8000000000000000ULL) == 0);
>         return reinterpret_cast<void*>(asBits_ << 1);
> #endif
>     }

There are at least following issues:
  * there are not getPrivate* methods (but toPrivate*)
  * it says 64-bit payload, but:
    * on 32-bit it only supports 32-bit
      (just wording issue?)
    * on 64-bit, it requirtes the lowest bit to be 0
      (pointer to 1-byte alignment things can fail this requirement, see bug 1505902)
  * on 64-bit the result of (ptr >> 1) needs to match isDouble(),
    which means, the pointer should be <= 0xfff00001fffffffe
    (almost all arbitrary pointers on supported 64-bit arch matches tho)

https://searchfox.org/mozilla-central/rev/06d5d5ae4396be85f26e8548323ee6c12e7bce4e/js/public/Value.h#621-627
>     bool isDouble() const {
> #if defined(JS_NUNBOX32)
>         return uint32_t(toTag()) <= uint32_t(JSVAL_TAG_CLEAR);
> #elif defined(JS_PUNBOX64)
>         return (asBits_ | mozilla::DoubleTypeTraits::kSignBit) <= JSVAL_SHIFTED_TAG_MAX_DOUBLE;
> #endif
>     }

Comment 1

[Iain Ireland [:iain]]

I fixed these issues as part of bug 1505902.