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Re: [RFC PATCH, i386]: Improve STV pass by correcting the cost of moves to/from XMM reg
On Mon, Aug 26, 2019 at 1:46 PM Richard Biener <rguenther@suse.de> wrote:
>
> On Fri, 23 Aug 2019, Uros Bizjak wrote:
>
> > On Fri, Aug 23, 2019 at 1:52 PM Richard Biener <rguenther@suse.de> wrote:
> > >
> > > On Fri, 23 Aug 2019, Uros Bizjak wrote:
> > >
> > > > This is currently a heads-up patch that removes the minimum limitation
> > > > of cost of moves to/from XMM reg. The immediate benefit is the removal
> > > > of mismatched spills, caused by subreg usage.
> > > >
> > > > *If* the patch proves to be beneficial (as in "doesn't regress
> > > > important benchmarks"), then we should be able to un-hide the
> > > > inter-regset moves from RA and allow it to collapse some moves. As an
> > > > example, patched compiler removes a movd in gcc.target/i386/minmax-6.c
> > > > and still avoids mismatched spill.
> > > >
> > > > 2019-08-23 Uroš Bizjak <ubizjak@gmail.com>
> > > >
> > > > * config/i386/i386.c (ix86_register_move_cost): Do not
> > > > limit the cost of moves to/from XMM register to minimum 8.
> > > > * config/i386/i386-features.c
> > > > (general_scalar_chain::make_vector_copies): Do not generate
> > > > zeroing move from GPR to XMM register, use gen_move_insn
> > > > instead of gen_gpr_to_xmm_move_src.
> > > > (general_scalar_chain::convert_op): Ditto.
> > > > (gen_gpr_to_xmm_move_src): Remove.
> > > >
> > > > The patch was bootstrapped and regression tested on x86_64-linux-gnu
> > > > {,-m32}, configured w/ and w/o -with-arch=ivybridge.
> > > >
> > > > The patch regresses PR80481 scan-asm-not (where the compiler generates
> > > > unrelated XMM spill on register starved x86_32). However, during the
> > > > analysis, I found that the original issue is not fixed, and is still
> > > > visible without -funrol-loops [1].
> > > >
> > > > [1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=80481#c10
> > > >
> > > > So, I'd wait for the HJ's benchmark results of the cost to/from XMM
> > > > change before proceeding with the patch.
> > >
> > > Otherwise looks good to me, it might clash (whitespace wise)
> > > with my STV "rewrite" though.
> > >
> > > We might want to adjust ix86_register_move_cost separately from
> > > the STV change to use regular moves though? Just to make bisection
> > > point to either of the two - STV "fallout"" is probably minor
> > > compared to fallout elsewhere...
> >
> > Yes, this is also my plan.
>
> Btw, when testing w/ the costing disabled I run into
>
> (insn 31 7 8 2 (set (subreg:V4SI (reg:SI 107) 0)
> (vec_merge:V4SI (vec_duplicate:V4SI (mem/c:SI (const:DI (plus:DI
> (symbol_ref:DI ("peakbuf") [flags 0x2] <var_decl 0x7efe8dbfeab0 peakbuf>)
> (const_int 400020000 [0x17d7d220]))) [1
> peakbuf.PeaksInBuf+0 S4 A64]))
> (const_vector:V4SI [
> (const_int 0 [0]) repeated x4
> ])
> (const_int 1 [0x1])))
>
> being not recognized (for the large immediate I guess). So when
> just doing
Hard to say without testcase, but I don't think this is valid memory
address. Can you see in dumps which insn is getting converted here?
> (set (reg:SI 107) (mem:SI ...))
>
> here we expect IRA to be able to allocate 107 into xmm, realizing
> it needs to reload the RHS first?
>
> For current code, is testing x86_64_general_operand like in the
> following the correct thing to do?
x86_64_general_operand will only limit immediates using
x86_64_immediate_operand, it will allow all memory_operands.
Uros.
> Thanks,
> Richard.
>
> Index: config/i386/i386-features.c
> ===================================================================
> --- config/i386/i386-features.c (revision 274926)
> +++ config/i386/i386-features.c (working copy)
> @@ -812,9 +812,15 @@ general_scalar_chain::convert_op (rtx *o
> else if (MEM_P (*op))
> {
> rtx tmp = gen_reg_rtx (GET_MODE (*op));
> + rtx tmp2 = *op;
>
> + if (!x86_64_general_operand (*op, smode))
> + {
> + tmp2 = gen_reg_rtx (smode);
> + emit_insn_before (gen_move_insn (tmp2, *op), insn);
> + }
> emit_insn_before (gen_rtx_SET (gen_rtx_SUBREG (vmode, tmp, 0),
> - gen_gpr_to_xmm_move_src (vmode,
> *op)),
> + gen_gpr_to_xmm_move_src (vmode,
> tmp2)),
> insn);
> *op = gen_rtx_SUBREG (vmode, tmp, 0);
>