On 9/28/23 15:43, Vineet Gupta wrote:
RISC-V suffers from extraneous sign extensions, despite/given the ABI
guarantee that 32-bit quantities are sign-extended into 64-bit registers,
meaning incoming SI function args need not be explicitly sign extended
(so do SI return values as most ALU insns implicitly sign-extend too.)
Existing REE doesn't seem to handle this well and there are various ideas
floating around to smarten REE about it.
RISC-V also seems to correctly implement middle-end hook PROMOTE_MODE
etc.
Another approach would be to prevent EXPAND from generating the
sign_extend in the first place which this patch tries to do.
The hunk being removed was introduced way back in 1994 as
5069803972 ("expand_expr, case CONVERT_EXPR .. clear the promotion flag")
This survived full testsuite run for RISC-V rv64gc with surprisingly no
fallouts: test results before/after are exactly same.
| | # of unexpected case / # of unique unexpected
case
| | gcc | g++ | gfortran |
| rv64imafdc_zba_zbb_zbs_zicond/| 264 / 87 | 5 / 2 | 72 / 12 |
| lp64d/medlow
Granted for something so old to have survived, there must be a valid
reason. Unfortunately the original change didn't have additional
commentary or a test case. That is not to say it can't/won't possibly
break things on other arches/ABIs, hence the RFC for someone to scream
that this is just bonkers, don't do this :-)
I've explicitly CC'ed Jakub and Roger who have last touched subreg
promoted notes in expr.cc for insight and/or screaming ;-)
Thanks to Robin for narrowing this down in an amazing debugging session
@ GNU Cauldron.
[ ... ]
So the data for Alpha was -- no change. I also put the patch into my
tester in the hopes that some target, any target would show a difference
in test results. It's churned about halfway through the embedded
targets so far with no regressions.
Given the data in your followup message, this clearly looks valuable and
so far we don't have any evidence Kenner's old change is useful or
necessary anymore.
I'm not (yet) comfortable committing this patch, I think the easy
avenues to getting a case where it's needed are not likely to prove
fruitful. So next steps here are for me to spend a bit more time trying
to understand what cases Kenner was trying to handle.
Jeff
