[Bug debug/25468] [3.4 Regression] -g makes g++ loop forever

["wilson at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #4 from wilson at gcc dot gnu dot org  2006-02-08 04:02 -------
The problem here is ASM_OUTPUT_ASCII.

The testcase is a big file with a lot of functions that have long function
names.  It is 4MB of C++ code, producing a 28MB .s file in about 15 seconds. 
If I add -Q and pipe the output to a file, I get 123MB, which means we have a
lot of strings just for the function names.  Adding -g, it takes about 20
minutes to compile the file, producing a .s file of about 108MB.  Of the extra
80MB much of it is going through ASM_OUTPUT_ASCII.

If you look at ASM_OUTPUT_ASCII, you see that the code uses putc to print one
character at a time while checking for characters that need to be quoted.  It
also throws in a lot of extra fprintf calls just to make this even slower.  So
we are trying to feed maybe 70MB through putc, and obviously, that is going to
take a lot of time.

If I hack in a definition of ASM_OUTPUT_ASCII that just uses 3 fputs calls,
then  it compiles in about 16 seconds.  This isn't quite right, as we are
missing quoting for special characters, but this clearly shows that the problem
is here in ASM_OUTPUT_ASCII.

There are some complicating factors here.  Some assemblers can't handle long
lines in the .ascii directive, so we deliberately break up the output into
lines less than 80 characters.  This can perhaps be dropped if we know we are
using GNU as.  We could maybe document the allowed line lengths for other
assemblers instead of assuming it is 80 characters.

ASM_OUTPUT_ASCII is defined in a lot of different files in subtly different
ways, and they all have this problem.

If we can assume that function names never contain special characters that need
quoting, then we can perhaps solve the problem by not using ASM_OUTPUT_ASCII
for this.  We could have a simpler and faster routine that doesn't worry about
the need to emit quoting characters.  This should be true for C.  I'm not sure
about C++ and the other languages.  This is probably a risky assumption to
make.

ASM_OUTPUT_ASCII could be faster if it scanned forward for the next special
character that needs quoting, then then emits everything up to the next special
character via puts (or whatever), modulo the allowed line length.


-- 

wilson at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
                 CC|                            |wilson at gcc dot gnu dot
                   |                            |org
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2006-02-08 04:02:15
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25468