Here is the code in fwprop.c:
/* OLD is a memory address. Return whether it is good to use NEW instead,
for a memory access in the given MODE. */
static bool
should_replace_address (rtx old_rtx, rtx new_rtx, enum machine_mode mode,
addr_space_t as, bool speed)
{
int gain;
if (rtx_equal_p (old_rtx, new_rtx)
|| !memory_address_addr_space_p (mode, new_rtx, as))
return false;
/* Copy propagation is always ok. */
if (REG_P (old_rtx) && REG_P (new_rtx))
return true;
* /* Prefer the new address if it is less expensive. */
gain = (address_cost (old_rtx, mode, as, speed)
- address_cost (new_rtx, mode, as, speed));
/* If the addresses have equivalent cost, prefer the new address
if it has the highest `set_src_cost'. That has the potential of
eliminating the most insns without additional costs, and it
is the same that cse.c used to do. */
if (gain == 0)
gain = set_src_cost (new_rtx, speed) - set_src_cost (old_rtx, speed);
return (gain > 0);*
}
According to the annotation, the 'return (gain > 0)' shouldn't be 'return
(gain >= 0)' ?
Here is the case for forward_addr.
insn set r155
plus r167 + 32
insn set mem (155)
r188
insn set mem (plus r155 + 8)
r189
......
If it is handled by the original code,
the result will be:
insn set r155
plus r167 + 32
insn set mem (r167 + 32)
r188
insn set mem (plus r155 + 8)
r189
However it is expected to be:
insn set mem (r167 + 32)
r188
insn set mem (plus r167 + 40)
r189
As the cost of 'addr r155' + 8 is equal to 'addr r167 + 40', so I think that
we should preffer to take the new addr, technically will be profitable ???
Brs,
Peter Xu.
-----
Dying in the sun.
--
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