https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108844
Bug ID: 108844
Summary: sincos opportunity missed
Product: gcc
Version: 13.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: tkoenig at gcc dot gnu.org
Target Milestone: ---
Two related test cases (which do the same, but are handled differently).
This is code for calculating a Jacobian, a frequent task in solving
non-linear systems of equations. (I am using C instead of Fortran because
Fortran does not support fallthrough).
$ cat a.c
#include <math.h>
void f1 (double x, double y, double f[2], double fjac[2][2], int flag)
{
switch (flag)
{
case 1:
f[0] = x * y;
f[1] = sin(x)*y*y;
break;
case 2:
fjac[0][0] = y;
fjac[1][0] = cos(x)*y*y;
fjac[0][1] = x;
fjac[1][1] = 2*sin(x)*y;
break;
case 3:
f[0] = x * y;
f[1] = sin(x)*y*y;
fjac[0][0] = y;
fjac[1][0] = cos(x)*y*y;
fjac[0][1] = x;
fjac[1][1] = 2*sin(x)*y;
break;
default:
__builtin_unreachable();
}
}
$ cat b.c
#include <math.h>
void f1 (double x, double y, double f[2], double fjac[2][2], int flag)
{
switch (flag)
{
case 1:
case 3:
f[0] = x * y;
f[1] = sin(x)*y*y;
if (flag != 3)
break;
/* Fallthrough */
case 2:
fjac[0][0] = y;
fjac[1][0] = cos(x)*y*y;
fjac[0][1] = x;
fjac[1][1] = 2*sin(x)*y;
break;
default:
__builtin_unreachable();
}
}
$ gcc -O3 -S a.c b.c
a.s looks good for flag=3:
leaq 64(%rsp), %rsi
leaq 72(%rsp), %rdi
movaps %xmm3, 48(%rsp)
movsd %xmm1, 32(%rsp)
movsd %xmm0, 24(%rsp)
call sincos
but the code for flag=2 looks like
cmpl $2, %edx
je .L2
[...]
.L2:
.cfi_restore_state
movaps %xmm3, 32(%rsp)
movsd %xmm1, 24(%rsp)
movsd %xmm0, (%rsp)
call cos
movsd (%rsp), %xmm2
movq %xmm0, %rbx
movapd %xmm2, %xmm0
call sin
b.s generates no call to sincos:
$ egrep '(sin|cos)' b.s
call sin
call sin
call cos
