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

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!

src/gen_dispatch.py

@@ -560,6 +560,23 @@ class Generator(object):
         self.outln('}')
         self.outln('')
 
+    def write_ifunc(self, func):
+        # Writes out the ifunc symbol that will have its PLT entry
+        # replaced with the resolved GL symbol when it's called.
+
+        # Tell the linker that even if we're in a static build, we need a PLT.
+        self.outln('__asm__(".type epoxy_{0}, @gnu_indirect_function");'.format(func.name));
+
+        # Tell the linker that epoxy_glWhatever() needs to be resolved
+        # by epoxy_ifunc_glWhatever().
+        self.outln('void *epoxy_ifunc_{0}(void) __asm__("epoxy_{0}");'.format(func.wrapped_name))
+
+        # Write out the actual epoxy_ifunc_glWhatever().
+        self.outln('{0} void *epoxy_ifunc_{1}(void)'.format(func.public, func.wrapped_name))
+        self.outln('{')
+        self.outln('    return epoxy_{0}_resolver();'.format(func.alias_name))
+        self.outln('}')
+
     def write_provider_enums(self):
         self.outln('enum {0}_provider {{'.format(self.target))
 
@@ -634,6 +651,22 @@ class Generator(object):
         self.outln('#include "epoxy/{0}.h"'.format(self.target))
         self.outln('')
 
+        # ifuncs are a newer-gcc and newer-dynamic-linker feature that
+        # let the library rewrite the application's PLT entry with our
+        # resolved function pointer.
+        #
+        # Note that dlsym() when we're called from the dynamic linker
+        # while it's loading libraries will break. So, libepoxy can't
+        # be loaded with RTLD_NOW when ifuncs are in use.  We also
+        # can't use ifuncs for static builds, since static builds
+        # resolve just like RTLD_NOW.
+        self.outln('#if defined(__PIC__) && defined(__linux__)')
+        self.outln('#define USING_IFUNCS 1')
+        self.outln('#else')
+        self.outln('#define USING_IFUNCS 0')
+        self.outln('#endif')
+        self.outln('')
+
         self.outln('struct dispatch_table {')
         for func in self.sorted_functions:
             # Aliases don't get their own slot, since they use a shared resolver.
@@ -646,9 +679,11 @@ class Generator(object):
         # bottom. (I want the function_ptr_resolver as the first
         # per-GL-call code, since it's the most interesting to see
         # when you search for the implementation of a call)
+        self.outln('#if !USING_IFUNCS')
         self.outln('static inline struct dispatch_table *')
         self.outln('get_dispatch_table(void);')
         self.outln('')
+        self.outln('#endif')
 
         self.write_provider_enums()
         self.write_provider_enum_strings()
@@ -658,6 +693,8 @@ class Generator(object):
             if not func.alias_func:
                 self.write_function_ptr_resolver(func)
 
+        self.outln('#if !USING_IFUNCS')
+
         for func in self.sorted_functions:
             if not func.alias_func:
                 self.write_dispatch_table_rewrite_stub(func)
@@ -681,6 +718,13 @@ class Generator(object):
         self.outln('}')
         self.outln('')
 
+        self.outln('#else /* !USING_IFUNCS */')
+
+        for func in self.sorted_functions:
+            self.write_ifunc(func)
+
+        self.outln('#endif /* !USING_IFUNCS */')
+
 argparser = argparse.ArgumentParser(description='Generate GL dispatch wrappers.')
 argparser.add_argument('files', metavar='file.xml', nargs='+', help='GL API XML files to be parsed')
 argparser.add_argument('--dir', metavar='dir', required=True, help='Destination directory')