Re: gcse pass: expression hash table

[Tarun Kawatra <tarun at cse dot iitb dot ac dot in>]

> My assumption here was that if I gave you a few pointers, you would try
> to debug the problem yourself.  If you want someone else to debug it for
> you, then you need to give much better info.  See for instance
>    http://gcc.gnu.org/bugs.html
> which gives info on how to properly report a bug.  I have the target and
> gcc version, but I need a testcase, compiler options, and perhaps other
> info.

I will take this into consideration now onwards.

The test case I am using (for multiplication expression is)
----------------------------
#include<stdio.h>

void foo();

int main()
{
        foo();
}
void foo()
{
        int a, b, c;
        int cond;

        scanf(" %d %d %d", &a, &b, &cond);
        if( cond )
                c = a * b;
        c = a * b;
        printf("Value of C is %d", c);
}
------------------------------
and for plus, a*b replaced by a+b everywhere.

I am compiling it as

gcc --param max-gcse-passes=2 -dF -dG -O3 filename.c

The reason for max-gcse-passes=2 is that in first pass a+b kind of 
expressions will be using different sets of pseudo registers at first and 
second occurance of a+b. After one gcse pass, both will become same 
(because of intermediate constant/copy propagation passes).
Then a+b gets optimized. As can be seen from dumps

filename.c.07.addressof
and filename.c.08.gcse

A part of expression hash table for program containing plus is

Expression hash table (11 buckets, 11 entries)

Index 0 (hash value 3)
  (plus:SI (reg/f:SI 20 frame)
    (const_int -4 [0xfffffffc]))

	....

Index 8 (hash value 1)
  (mem/f:SI (plus:SI (reg/f:SI 20 frame)
        (const_int -8 [0xfffffff8])) [2 b+0 S4 A32])
Index 9 (hash value 6)
  (plus:SI (reg:SI 78 [ a ])
    (reg:SI 79 [ b ]))
Index 10 (hash value 10)
  (plus:SI (reg:SI 80 [ a ])
    (reg:SI 81 [ b ]))

Which clearly shows that clobbering CC in a+b is being ignored if the 
expressions which requires to be inserted will not be containing 
clobbering of CC.

> How do you know that adds are getting optimized?  Did you judge this by

I am looking at dump files.

> looking at one of the dump files, or looking at the assembly output?
> Maybe you are looking at the wrong thing, or misunderstanding what you
> are looking at?  You need to give more details here.

Regards,
-tarun