TZUTC: -0800
MSGID: 59111.sync_sys@1:103/705 2dbda0c2
PID: Synchronet 3.21a-Linux master/48922a15c Dec 17 2025 GCC 12.2.0
TID: SBBSecho 3.34-Linux master/8bb133aa7 Dec 30 2025 GCC 12.2.0
BBSID: VERT
CHRS: UTF-8 4
FORMAT: flowed
https://gitlab.synchro.net/main/sbbs/-/issues/685#note_8070

So, just took a look... while it's easy enough to extend the value to 48-bits
by taking a bit from the tag, it appears that newer version of SpiderMonkey do
*not* actually support more than 47 value bits

ie: `TAG_SHIFT` is still 47, and `bitsFromTagAndPayload()` is
```C++
  static constexpr uint64_t bitsFromTagAndPayload(JSValueTag tag,
                                                  PayloadType payload) {
    return (uint64_t(tag) << JSVAL_TAG_SHIFT) | payload;
  }
```

So I'm not sure how Firefox and GNOME are getting away with this, though the
`setarch` manpage perhaps has a hint:

```

       -L, --addr-compat-layout
           Provide legacy virtual address space layout. Use when the
           program binary does not have PT_GNU_STACK ELF header. Turns on
           ADDR_COMPAT_LAYOUT.
```

Is there some way to set `ADDR_COMPAT_LAYOUT` in the `PT_GNU_STACK` header via
linker flags?
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