On Wed, May 24, 2023 at 7:47 AM Alexandre Oliva via Gcc-patches <gcc-patches@gcc.gnu.org> wrote: > > > MOVE_MAX on x86* used to accept up to 16 bytes, even without SSE, > which enabled inlining of small memmove by loading and then storing > the entire range. After the "x86: Update piecewise move and store" > r12-2666 change, memmove of more than 4 bytes would not be inlined in > gimple_fold_bultin_memory_op, failing the expectations of a few tests. > > I can see how lowering it for MOVE_MAX_PIECES can get us better > codegen decisions overall, but surely inlining memmove with 2 32-bit > loads and stores is better than an outline call that requires setting > up 3 arguments. I suppose even 3 or 4 could do better. But maybe it > is gimple_fold_builtin_memory_op that needs tweaking?
gimple_fold_builtin_memory_op tries to expand the call to a single load plus a single store so we can handle overlaps by first loading everything to registers and then storing: /* If we can perform the copy efficiently with first doing all loads and then all stores inline it that way. Currently efficiently means that we can load all the memory into a single integer register which is what MOVE_MAX gives us. */ using DImode on i?86 without SSE means we eventually perform two loads and two stores which means we need two registers available. That might not be an issue on x86_64 doing 16 bytes with two DImode ops (and -mno-sse) since there's plenty of regs available. So I think if we want to expand this further at the GIMPLE level we should still honor MOVE_MAX but eventually emit multiple loads/stores honoring the MOVE_MAX_PIECES set of constraints there and avoid expanding to sequences where we cannot interleave the loads/stores (aka for the memmove case). > Anyhow, this patch raises MOVE_MAX back a little for non-SSE targets, > while preserving the new value for MOVE_MAX_PIECES. > > Bootstrapped on x86_64-linux-gnu. Also tested on ppc- and x86-vx7r2 > with gcc-12. > > for gcc/ChangeLog > > * config/i386/i386.h (MOVE_MAX): Rename to... > (MOVE_MAX_VEC): ... this. Add NONVEC parameter, and use it as > the last resort, instead of UNITS_PER_WORD. > (MOVE_MAX): Reintroduce in terms of MOVE_MAX_VEC, with > 2*UNITS_PER_WORD. > (MOVE_MAX_PIECES): Likewise, but with UNITS_PER_WORD. > --- > gcc/config/i386/i386.h | 6 ++++-- > 1 file changed, 4 insertions(+), 2 deletions(-) > > diff --git a/gcc/config/i386/i386.h b/gcc/config/i386/i386.h > index c7439f89bdf92..5293a332a969a 100644 > --- a/gcc/config/i386/i386.h > +++ b/gcc/config/i386/i386.h > @@ -1801,7 +1801,9 @@ typedef struct ix86_args { > is the number of bytes at a time which we can move efficiently. > MOVE_MAX_PIECES defaults to MOVE_MAX. */ > > -#define MOVE_MAX \ > +#define MOVE_MAX MOVE_MAX_VEC (2 * UNITS_PER_WORD) > +#define MOVE_MAX_PIECES MOVE_MAX_VEC (UNITS_PER_WORD) > +#define MOVE_MAX_VEC(NONVEC) \ > ((TARGET_AVX512F \ > && (ix86_move_max == PVW_AVX512 \ > || ix86_store_max == PVW_AVX512)) \ > @@ -1813,7 +1815,7 @@ typedef struct ix86_args { > : ((TARGET_SSE2 \ > && TARGET_SSE_UNALIGNED_LOAD_OPTIMAL \ > && TARGET_SSE_UNALIGNED_STORE_OPTIMAL) \ > - ? 16 : UNITS_PER_WORD))) > + ? 16 : (NONVEC)))) > > /* STORE_MAX_PIECES is the number of bytes at a time that we can store > efficiently. Allow 16/32/64 bytes only if inter-unit move is enabled > > -- > Alexandre Oliva, happy hacker https://FSFLA.org/blogs/lxo/ > Free Software Activist GNU Toolchain Engineer > Disinformation flourishes because many people care deeply about injustice > but very few check the facts. Ask me about <https://stallmansupport.org>