Hi
I am using a cross compiler for Renesas H8S. In a few places it
generates really bad code. Given the following program:
struct s {
char a, b;
char c[11];
} x[2];
void test(int n)
{
struct s *sp = &x[n];
sp->a = 1;
sp->b = 1;
}
I would expect that the pointer "sp" is calculated once and reused to
access the fields "a" and "b". But instead the pointer is recalculated
for each access. This generates a lot of extra code, including calls to
__mulhi3. I have tested with gcc 8.2 and 9.2 and with different
optimization levels (-O1, -O2, -Os) all with the same result. With -O0
"sp" is only calculated once and kept as a variable on the stack but the
rest of the code is not as good as it could be. The best work around
seems to be to declare "sp" as volatile: "struct s *volatile sp =
&x[n];". Then "sp" is only calculated once and kept on the stack and the
surrounding code can be optimized.
So my question is: where should I start looking for a fix to this? The
other targets I tried (ARM, x86, x86_64) behave as expected and
calculates the pointer once and keeps it in a register.
/Mikael
