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FYI: micro-optimize producer checks
- From: Tom Tromey <tromey at redhat dot com>
- To: GDB Development <gdb at sourceware dot org>
- Date: Fri, 09 Mar 2012 13:17:11 -0700
- Subject: FYI: micro-optimize producer checks
I'm checking this in.
This is a second CU expansion performance micro-optimization.
As before, my test case was 'gdb -batch -readnow gdb'.
I neglected to mention in the earlier note that these tests and numbers
were done with a default ("-g -O2") build of gdb, using the Fedora 16
If you instead use '-g3', you will also see a big slowdown from a call
to complaint (!) in dwarf_decode_macro_bytes (it is actually dcgettext
which shows up in the profile). This is related to a gcc bug (I
surmise, I didn't dig into it); but regardless, I think that this means
that the complaint system in gdb needs a fix. I didn't do this today,
Anyway. I was surprised to find that gdb was spending a fair amount of
time in sscanf during CU expansion. This was caused by repeated calls
The fix is to cache this information. This is space-neutral but takes
us from 2:36 to 2:23 on the test case. This is another 5% improvement.
Built and regtested on x86-64 Fedora 16.
2012-03-09 Tom Tromey <firstname.lastname@example.org>
* dwarf2read.c (struct dwarf2_cu) <checked_producer,
producer_is_gxx_lt_4_6>: New fields.
(producer_is_gxx_lt_4_6): Use and update producer cache fields.
RCS file: /cvs/src/src/gdb/dwarf2read.c,v
retrieving revision 1.621
diff -u -r1.621 dwarf2read.c
--- dwarf2read.c 9 Mar 2012 20:06:18 -0000 1.621
+++ dwarf2read.c 9 Mar 2012 20:13:10 -0000
@@ -383,6 +383,13 @@
any location list and still facing inlining issues if handled as
unoptimized code. For a future better test see GCC PR other/32998. */
unsigned int has_loclist : 1;
+ /* These cache the results of producer_is_gxx_lt_4_6.
+ CHECKED_PRODUCER is set if PRODUCER_IS_GXX_LT_4_6 is valid. This
+ information is cached because profiling CU expansion showed
+ excessive time spent in producer_is_gxx_lt_4_6. */
+ unsigned int checked_producer : 1;
+ unsigned int producer_is_gxx_lt_4_6 : 1;
/* Persistent data held for a compilation unit, even when not
@@ -6826,6 +6833,7 @@
const char *cs;
int major, minor, release;
+ int result = 0;
if (cu->producer == NULL)
@@ -6841,26 +6849,33 @@
+ if (cu->checked_producer)
+ return cu->producer_is_gxx_lt_4_6;
/* Skip any identifier after "GNU " - such as "C++" or "Java". */
if (strncmp (cu->producer, "GNU ", strlen ("GNU ")) != 0)
/* For non-GCC compilers expect their behavior is DWARF version
- return 0;
- cs = &cu->producer[strlen ("GNU ")];
- while (*cs && !isdigit (*cs))
- if (sscanf (cs, "%d.%d.%d", &major, &minor, &release) != 3)
- /* Not recognized as GCC. */
- return 0;
+ cs = &cu->producer[strlen ("GNU ")];
+ while (*cs && !isdigit (*cs))
+ if (sscanf (cs, "%d.%d.%d", &major, &minor, &release) != 3)
+ /* Not recognized as GCC. */
+ result = major < 4 || (major == 4 && minor < 6);
- return major < 4 || (major == 4 && minor < 6);
+ cu->checked_producer = 1;
+ cu->producer_is_gxx_lt_4_6 = result;
+ return result;
/* Return the default accessibility type if it is not overriden by