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 I scoured my old gcc2 archives to see if there was anything that
might hint as to why this was changed. Sadly (but not unexpectedly),
nothing. The relevant ChangeLog entry is;
Fri Jul 8 11:46:50 1994 Richard Kenner (ken...@vlsi1.ultra.nyu.edu)
* varasm.c (record_constant_rtx, force_const_mem): Ensure everything
is in saveable_obstack, not current_obstack.
* combine.c (force_to_mode): OP_MODE must be MODE if MODE and
mode of X are of different classes.
(nonzero_bits, num_sign_bit_copies): Say nothing known for
floating-point modes.
* function.c (instantiate_virtual_regs_1, case SET):
If DEST is virtual_stack_vars_rtx, replace with hardware
frame pointer.
* expr.c (expand_expr, case CONVERT_EXPR): If changing signedness
and we have a promoted SUBREG, clear the promotion flag.
* c-decl.c (finish_decl): Put RTL and other stuff in
permanent_obstack if DECL is.
* combine.c (gen_unary): Add new arg, OP0_MODE.
All callers changed.
So standard practice back then was to re-use the header and have a blank
line between conceptual changes if the same author made a series of
changes. So it's reasonable to assume the expr.c change was considered
independent of the other changes.
At that particular time I think Kenner was mostly focused on the alpha
and ppc ports, but I think he was also still poking around with romp and
a29k. I think romp is an unlikely target for this because it didn't
promote modes and it wasn't even building for several months
(April->late July).
PPC and a29k were both 32 bit ports and while they did promotions, I
would hazard a guess the alpha was actually more sensitive to this
stuff. Which suggests a possible path forward.
I can bootstrap & regression test alpha using QEMU user mode emulation.
So we might be able to trigger something that way. It'll take some
time, but might prove fruitful.
Jeff
