On Fri, 14 Jul 2023, Uros Bizjak wrote:
> On Fri, Jul 14, 2023 at 10:31?AM Richard Biener <rguent...@suse.de> wrote:
> >
> > On Fri, 14 Jul 2023, Uros Bizjak wrote:
> >
> > > cprop1 pass does not consider paradoxical subreg and for (insn 22) claims
> > > that it equals 8 elements of HImodeby setting REG_EQUAL note:
> > >
> > > (insn 21 19 22 4 (set (reg:V4QI 98)
> > > (mem/u/c:V4QI (symbol_ref/u:DI ("*.LC1") [flags 0x2]) [0 S4
> > > A32])) "pr110206.c":12:42 1530 {*movv4qi_internal}
> > > (expr_list:REG_EQUAL (const_vector:V4QI [
> > > (const_int -52 [0xffffffffffffffcc]) repeated x4
> > > ])
> > > (nil)))
> > > (insn 22 21 23 4 (set (reg:V8HI 100)
> > > (zero_extend:V8HI (vec_select:V8QI (subreg:V16QI (reg:V4QI 98) 0)
> > > (parallel [
> > > (const_int 0 [0])
> > > (const_int 1 [0x1])
> > > (const_int 2 [0x2])
> > > (const_int 3 [0x3])
> > > (const_int 4 [0x4])
> > > (const_int 5 [0x5])
> > > (const_int 6 [0x6])
> > > (const_int 7 [0x7])
> > > ])))) "pr110206.c":12:42 7471
> > > {sse4_1_zero_extendv8qiv8hi2}
> > > (expr_list:REG_EQUAL (const_vector:V8HI [
> > > (const_int 204 [0xcc]) repeated x8
> > > ])
> > > (expr_list:REG_DEAD (reg:V4QI 98)
> > > (nil))))
> > >
> > > We rely on the "undefined" vals to have a specific value (from the earlier
> > > REG_EQUAL note) but actual code generation doesn't ensure this (it doesn't
> > > need to). That said, the issue isn't the constant folding per-se but that
> > > we do not actually constant fold but register an equality that doesn't
> > > hold.
> > >
> > > PR target/110206
> > >
> > > gcc/ChangeLog:
> > >
> > > * fwprop.cc (contains_paradoxical_subreg_p): Move to ...
> > > * rtlanal.cc (contains_paradoxical_subreg_p): ... here.
> > > * rtlanal.h (contains_paradoxical_subreg_p): Add prototype.
> > > * cprop.cc (try_replace_reg): Do not set REG_EQUAL note
> > > when the original source contains a paradoxical subreg.
> > >
> > > gcc/testsuite/ChangeLog:
> > >
> > > * gcc.dg/torture/pr110206.c: New test.
> > >
> > > Bootstrapped and regression tested on x86_64-linux-gnu {,-m32}.
> > >
> > > OK for mainline and backports?
> >
> > OK.
> >
> > I think the testcase can also run on other targets if you add
> > dg-additional-options "-w -Wno-psabi", all generic vector ops
> > should be lowered if not supported.
>
> True, but with lowered vector ops, the test would not even come close
> to the problem. The problem is specific to generic vector ops, and can
> be triggered only when paradoxical subregs are used to implement
> (partial) vector modes. This is the case on x86, where partial vectors
> are now heavily used, and even there we need the latest vector ISA
> enabled to trip the condition.
>
> The above is the reason that dg-torture is used, with the hope that
> the runtime failure will trip when testsuite is run with specific
> target options.
I see. I'm fine with this then though moving to gcc.target/i386
with appropriate triggering options and a dg-require for runtime
support would also work.
Richard.
