Hello! I noticed an inefficiency in libGL.so, so I thought I'd take a
stab at fixing it. This is my first patch submitted to mesa-dev, so
if I'm doing anything dumb, let me know. I can't use git send-email,
but I've formatted the patch using git format-patch, which should
hopefully produce similar output. The patch text (below) describes
the inefficiency and the improvement.
From 0abde226eed1b9f6052193f36f6cdc060698f621 Mon Sep 17 00:00:00 2001
From: Arlie Davis <arl...@google.com>
Date: Tue, 15 Sep 2015 09:58:34 -0700
Subject: [PATCH] This patch significantly reduces the size of the libGL.so
binary. It does not change the (externally visible) behavior of libGL.so at
all.
gl_gentable.py generates a function, _glapi_create_table_from_handle.
This function allocates a large dispatch table, consisting of 1300 or so
function pointers, and fills this dispatch table by doing symbol lookups
on a given shared library. Previously, gl_gentable.py would generate a
single, very large _glapi_create_table_from_handle function, with a short
cluster of lines for each entry point (function). The idiom it generates
was a NULL check, a call to snprintf, a call to dlsym / GetProcAddress,
and then a store into the dispatch table. Since this function processes
a large number of entry points, this code is duplicated many times over.
We can encode the same information much more compactly, by using a lookup
table. The previous total size of _glapi_create_table_from_handle on x64
was 125848 bytes. By using a lookup table, the size of
_glapi_create_table_from_handle (and the related lookup tables) is reduced
to 10840 bytes. In other words, this enormous function is reduced by 91%.
The size of the entire libGL.so binary (measured when stripped) itself drops
by 15%.
So the purpose of this change is to reduce the binary size, which frees up
disk space, memory, etc.
---
src/mapi/glapi/gen/gl_gentable.py | 56 ++++++++++++++++++++++++++++-----------
1 file changed, 40 insertions(+), 16 deletions(-)
diff --git a/src/mapi/glapi/gen/gl_gentable.py
b/src/mapi/glapi/gen/gl_gentable.py
index 1b3eb72..2563b6b 100644
--- a/src/mapi/glapi/gen/gl_gentable.py
+++ b/src/mapi/glapi/gen/gl_gentable.py
@@ -113,6 +113,9 @@ __glapi_gentable_set_remaining_noop(struct _glapi_table
*disp) {
dispatch[i] = p.v;
}
+"""
+
+footer = """
struct _glapi_table *
_glapi_create_table_from_handle(void *handle, const char *symbol_prefix) {
struct _glapi_table *disp = calloc(_glapi_get_dispatch_table_size(),
sizeof(_glapi_proc));
@@ -123,27 +126,27 @@ _glapi_create_table_from_handle(void *handle, const char
*symbol_prefix) {
if(symbol_prefix == NULL)
symbol_prefix = "";
-"""
-footer = """
- __glapi_gentable_set_remaining_noop(disp);
-
- return disp;
-}
-"""
+ /* Note: This code relies on _glapi_table_func_names being sorted by the
+ entry point index of each function. */
+ for (int func_index = 0; func_index < GLAPI_TABLE_COUNT; ++func_index) {
+ const char* name = _glapi_table_func_names[func_index];
+ void ** procp = &((void **)disp)[func_index];
-body_template = """
- if(!disp->%(name)s) {
- void ** procp = (void **) &disp->%(name)s;
- snprintf(symboln, sizeof(symboln), "%%s%(entry_point)s",
symbol_prefix);
+ snprintf(symboln, sizeof(symboln), \"%s%s\", symbol_prefix, name);
#ifdef _WIN32
*procp = GetProcAddress(handle, symboln);
#else
*procp = dlsym(handle, symboln);
#endif
}
+ __glapi_gentable_set_remaining_noop(disp);
+
+ return disp;
+}
"""
+
class PrintCode(gl_XML.gl_print_base):
def __init__(self):
@@ -180,12 +183,33 @@ class PrintCode(gl_XML.gl_print_base):
def printBody(self, api):
- for f in api.functionIterateByOffset():
- for entry_point in f.entry_points:
- vars = { 'entry_point' : entry_point,
- 'name' : f.name }
- print body_template % vars
+ # Determine how many functions have a defined offset.
+ func_count = 0
+ for f in api.functions_by_name.itervalues():
+ if f.offset != -1:
+ func_count += 1
+
+ # Build the mapping from offset to function name.
+ funcnames = [None] * func_count
+ for f in api.functions_by_name.itervalues():
+ if f.offset != -1:
+ if not (funcnames[f.offset] is None):
+ raise Exception("Function table has more than one function
with same offset (offset %d, func %s)" % (f.offset, f.name))
+ funcnames[f.offset] = f.name
+
+ # Check that the table has no gaps. We expect a function at every
offset,
+ # and the code which generates the table relies on this.
+ for i in range(0, func_count):
+ if funcnames[i] is None:
+ raise Exception("Function table has no function at offset %d"
% (i))
+
+ print "#define GLAPI_TABLE_COUNT %d" % func_count
+ print "static const char* const
_glapi_table_func_names[GLAPI_TABLE_COUNT] = {"
+ for i in range(0, func_count):
+ print " /* %5d */ \"%s\"," % (i, funcnames[i])
+ print "};"
+
return
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