compiling the following code:
for ( i = 0; i < cnt; )
{
x1 = 2.0 * x - 1.;
if ( x1 < 1.0 )
{
b[i] = exp( x1 ) * cos( x1 );
i = i + 3;
x = x*2.;
}
else // if ( x1 >= 1. )
{
a[i] = sqrt( x1 ) * log( 1 / x1 );
i = i + 2;
x = x/2.;
}
}
with “gcc -O2 -S” generates
jmp .L6
.L19:
movapd %xmm3, %xmm0
movsd %xmm3, 24(%rsp)
addl $3, %ebx
call __exp_finite
movsd 24(%rsp), %xmm3
movsd %xmm0, 16(%rsp)
movapd %xmm3, %xmm0
call cos
mulsd 16(%rsp), %xmm0
cmpl %ebx, %r12d
movsd 8(%rsp), %xmm2
movapd %xmm2, %xmm1
movsd %xmm0, (%r14,%rbp,8)
jbe .L18
.L6:
movapd %xmm1, %xmm2
movsd .LC1(%rip), %xmm4
movl %ebx, %ebp
addsd %xmm1, %xmm2
comisd %xmm2, %xmm4
movapd %xmm2, %xmm3
movsd %xmm2, 8(%rsp)
subsd .LC0(%rip), %xmm3
ja .L19
movsd .LC0(%rip), %xmm0
sqrtsd %xmm3, %xmm6
movsd %xmm1, 16(%rsp)
addl $2, %ebx
divsd %xmm3, %xmm0
movsd %xmm6, 8(%rsp)
call __log_finite
mulsd 8(%rsp), %xmm0
cmpl %ebx, %r12d
movsd 16(%rsp), %xmm1
mulsd .LC2(%rip), %xmm1
movsd %xmm0, 0(%r13,%rbp,8)
ja .L6
.L18:
How to force the compiler to generate assembly loop that is “similar” to
the C code specified:
one entry, one exit.
