On 4/19/23 20:46, liuhongt via Gcc-patches wrote:
1547 /* If this insn loads a parameter from its stack slot, then it
1548 represents a savings, rather than a cost, if the parameter is
1549 stored in memory. Record this fact.
1550
1551 Similarly if we're loading other constants from memory (constant
1552 pool, TOC references, small data areas, etc) and this is the only
1553 assignment to the destination pseudo.
At that time, preferred regclass is unknown, and GENERAL_REGS is used to
record memory move cost, but it's not accurate especially for large vector
modes, i.e. 512-bit vector in x86 which would most probably allocate with
SSE_REGS instead of GENERAL_REGS. Using GENERAL_REGS here will overestimate
the cost of this load and make RA propagate the memeory operand into many
consume instructions which causes worse performance.
For this case GENERAL_REGS was used in GCC practically all the time.
You can check this in the old regclass.c file (existing until IRA
introduction).
But I guess it is ok to use NO_REGS for this to promote more usage of
registers instead of equiv memory and as a lot of code was changed since
then (the old versions of GCC even did not support vector regs).
Although it would be nice to do some benchmarking (SPEC is preferable)
for such kind of changes.
On the other hand, I expect that any performance regression (if any)
will be reported anyway.
The patch is ok for me. You can commit it into the trunk.
Thank you for addressing this issue.
Fortunately, NO_REGS is used to record the best scenario, so the patch uses
NO_REGS instead of GENERAL_REGS here, it could help RA in PR108707.
Bootstrapped and regtested on x86_64-pc-linux-gnu{-m32,}
and aarch64-linux-gnu.
Ok for trunk?
gcc/ChangeLog:
PR rtl-optimization/108707
* ira-costs.cc (scan_one_insn): Use NO_REGS instead of
GENERAL_REGS when preferred reg_class is not known.
gcc/testsuite/ChangeLog:
* gcc.target/i386/pr108707.c: New test.
