[RFC] expr: don't clear SUBREG_PROMOTED_VAR_P flag for a promoted subreg [target/111466]

[Jeff Law]

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  (kenner@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