https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98596
Jim Wilson <wilson at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Status|UNCONFIRMED |NEW
Last reconfirmed| |2021-01-14
CC| |wilson at gcc dot gnu.org
--- Comment #1 from Jim Wilson <wilson at gcc dot gnu.org> ---
Part of the problem seems to be the use of volatile, as we disable
optimizations inside volatile operations, and both constants are used inside
volatile operations.
But a bigger issue seems to be how we calculate constant costs. In
riscv_rtx_costs we have
/* If the constant is likely to be stored in a GPR, SETs of
single-insn constants are as cheap as register sets; we
never want to CSE them. */
if (cost == 1 && outer_code == SET)
*total = 0;
which tells the compiler that constants are cheaper than registers. If I
change that to "*total = 1;" then the two constants get optimized. Changing
this cost means we will likely extend register lifetimes and increase register
pressure, which may reduce performance in some applications. We would need a
lot of tersting to see what happens. We are already computing a cost of 0 for
constants in the arithmetic immediate range, so setting costs to 0 here seems
unnecessary, but it is hard to predict what might happen with this change.
There might be something else I'm missing here.
