> Posting some random numbers without a test-case and precise command line
> parameters for both compilers makes the numbers useless, IMHO. You also
> only mention instruction counts. Have you actually benchmarked the
> resulting code? CPUs are complicated and what you might perceive as worse
> code might actually be superior thanks to scheduling and internal CPU
> parallelism etc.
Thanks for reminding.
After some investigation, I could demonstrate the issue by following
piece of code:
-------------------------------------begin here-------------------
extern int *p[5];
# define REAL_RADIX_2 24
# define REAL_MUL_2(x, y) (((long long)(x) * (long long)(y)) >>
REAL_RADIX_2)
void func(int *b1, int *b2)
{
int c0 = p[3][0];
int c1 = p[3][1];
b2[0x18] = b1[0x18] + b1[0x1B];
b2[0x1B] = REAL_MUL_2((b1[0x18] - b1[0x1B]) , c0);
b2[0x19] = b1[0x19] + b1[0x1A];
b2[0x1A] = REAL_MUL_2((b1[0x19] - b1[0x1A]) , c1);
b2[0x1C] = b1[0x1C] + b1[0x1F];
b2[0x1F] = REAL_MUL_2((b1[0x1F] - b1[0x1C]) , c0);
b2[0x1D] = b1[0x1D] + b1[0x1E];
b2[0x1E] = REAL_MUL_2((b1[0x1E] - b1[0x1D]) , c1);
}
-------------------------------------cut here-------------------
It seems GCC4.3.4 always expands the long long multiplication into
three long multiplications, like
-------------------------------------begin here-------------------
# b2[0x1A] = REAL_MUL_2((b1[0x19] - b1[0x1A]) , c1);
lw $6,104($4)
lw $2,100($4)
subu $2,$2,$6
mult $11,$2
sra $6,$2,31
madd $6,$9
mflo $6
multu $2,$9
mfhi $3
addu $3,$6,$3
sll $6,$3,8
mflo $2
srl $7,$2,24
or $7,$6,$7
sw $7,104($5)
-------------------------------------cut here-------------------
while GCC3.4.4 treats the long long multiplication just like simple
ones, which generates only one
mult insn for each statement, like
-------------------------------------begin here-------------------
# b2[0x1A] = REAL_MUL_2((b1[0x19] - b1[0x1A]) , c1);
lw $2,100($4)
lw $7,104($4)
subu $3,$2,$7
mult $3,$9
mflo $6
mfhi $25
srl $15,$6,24
sll $24,$25,8
or $14,$15,$24
sw $14,104($5)
-------------------------------------cut here-------------------
In my understanding, It‘s not necessary using three mult insn to implement
long long mult, since the operands are converted from int type.
And as before, the compiling options are like "-march=mips32r2 -O3"
Thanks.
--
Best Regards.
