Use udis86 to walk x64 machine code in find_fast_cwd_pointer

[Corinna Vinschen]

On Mar 18 22:11, Jeremy Drake via Cygwin-patches wrote:
> On Tue, 18 Mar 2025, Corinna Vinschen wrote:
> 
> > Subdir of winsup/cygwin, probably.  What I'm most curious about is the
> > size it adds to the DLL.  I wonder if, say, an extra 32K is really
> > usefully spent, given it only checks a small part of ntdll.dll, and only
> > once per process tree, too.
> 
> I did this with msys-2.0.dll, but it shouldn't matter as a delta.
> all are stripped msys-2.0.dll size
> start:
> 3,246,118 bytes
> with udis86 vendored, but not called:
> 3,247,142 bytes
> with find_fast_cwd_pointer rewritten to use udis86:
> 3,328,550 bytes
> 
> (I know the second one isn't realistic, the linker could exclude unused
> code, I was just kind of curious)
> 
> This is with all the "translate to assembly text, intel or at&t syntax"
> and "table of strings for opcodes" stuff removed to try to save space,
> still a net increase of 82,432 bytes.

The DLL has currently a size of 3 Megs, optimzed, stripped.  82K are
two more allocation granularity slots, 51 instead of 49, about 2%.
So in reality you probably won't notice a difference in loading time
or memory pressure.

>   /* Next we search for the locking mechanism and perform a sanity check.
>      On Pre- (or Post-) Windows 8 we basically look for the
>      RtlEnterCriticalSection call.  Windows 8 does not call
>      RtlEnterCriticalSection.  The code manipulates the FastPebLock manually,
>      probably because RtlEnterCriticalSection has been converted to an inline
>      function.  Either way, we test if the code uses the FastPebLock. */

We can throw anything related to pre-8.1 out of the Window, if it helps
to reduce the code.

Corinna