Use ifuncs on Linux to avoid dispatch table overhead.

These tell the linker to generate GNU_IFUNC relocs that rewrite the
PLT entries in the user's address space to point to our resolved GL
function, so there's no extra function pointer.  It also, as a bonus,
cuts 400k out of the library.

This requires a toolchain from 2010 or so.  Unfortunately, it's going
to take a bit more investigation to find out what specific bits are
required.

Fixes #4

1 files changed, 18 insertions, 0 deletions

diff --git a/README.md b/README.md
index 57890e2..489943e 100644
--- a/README.md
+++ b/README.md
@@ -80,3 +80,21 @@ We had to solve some of GLEW's problems for piglit and solving them
 meant replacing every single piece of GLEW, so we built
 piglit-dispatch from scratch.  And since we wanted to reuse it in
 other GL-related projects, this is the result.
+
+Caveats
+-------
+
+* libepoxy isn't intended to be dlopen()ed.  On ifunc platforms,
+  RTLD_NOW would cause early resolution of all GL symbols, and the
+  following error from reentering the linker:
+
+```
+Inconsistency detected by ld.so: dl-open.c: 235: dl_open_worker:
+Assertion `_dl_debug_initialize (0, args->nsid)->r_state ==
+RT_CONSISTENT' failed!
+```
+
+* libepoxy isn't intended to be built statically.  It usess ifuncs on
+  linux, since it can avoid its dispatch table entirely in that case.
+  Building statically disables ifuncs, dropping you to a
+  higher-overhead path!