[Mesa-dev] [PATCH] configure.ac: Add support to enable read-only text segment on x86.

[Matt Turner]

On Sat, Aug 29, 2015 at 4:27 PM, Ilia Mirkin <imirkin at alum.mit.edu> wrote:
> Why isn't this the default? IOW should this be incompatible with some
> other option and/or set based on some other option?

Please don't top quote.

If I understand correctly, this option partially disables an
optimization in the dispatch table, but the options are still
distinct.

My understanding is that with the x86 assembly dispatch
implementation, either (or both...?) of

glapi_entrypoint.c:init_glapi_relocs()
entry_x86_tls.h:entry_patch_public()

patch .text to contain the TLS pointer, so that the expensive "movl
%gs:0x0, %eax" doesn't happen on each GL function call. But that of
course requires a writable .text section, which is frowned upon.

So --enable-glx-rts (I'd be happy to change the name to something less
cryptic) inhibits that, and instead does the expensive TLS look-up
(see src/mapi/glapi/gen/gl_x86_asm.py) on each GL call, but still uses
the assembly implementation.

The assembly implementation is able to jump directly to the _mesa_*
function implementation (that is, not a call instruction that pushes
more stuff on the stack), so I think even with --enable-glx-rts, the
assembly implementation is still better than the C implementation.

We'll see if idr reads his mail and can confirm my understanding.