On Thu, Aug 19, 2021 at 6:01 PM Roger Sayle <ro...@nextmovesoftware.com> wrote: > > > Doh! ENOPATCH. > > -----Original Message----- > From: Roger Sayle <ro...@nextmovesoftware.com> > Sent: 19 August 2021 16:59 > To: 'GCC Patches' <gcc-patches@gcc.gnu.org> > Subject: [x86_64 PATCH] Tweak -Os costs for scalar-to-vector pass. > > > Back in June I briefly mentioned in one of my gcc-patches posts that a > change that should have always reduced code size, would mysteriously > occasionally result in slightly larger code (according to CSiBE): > https://gcc.gnu.org/pipermail/gcc-patches/2021-June/573233.html > > Investigating further, the cause turns out to be that x86_64's > scalar-to-vector (stv) pass is relying on poor estimates of the size > costs/benefits. This patch tweaks the backend's compute_convert_gain method > to provide slightly more accurate values when compiling with -Os. > Compilation without -Os is (should be) unaffected. And for completeness, > I'll mention that the stv pass is a net win for code size so it's much > better to improve its heuristics than simply gate the pass on > !optimize_for_size. > > The net effect of this change is to save 1399 bytes on the CSiBE code size > benchmark when compiling with -Os. > > This patch has been tested on x86_64-pc-linux-gnu with "make bootstrap" > and "make -k check" with no new failures. > > Ok for mainline?
+ /* xor (2 bytes) vs. xorps (3 bytes). */ + if (src == const0_rtx) + igain -= COSTS_N_BYTES (1); + /* movdi_internal vs. movv2di_internal. */ + /* => mov (5 bytes) vs. movaps (7 bytes). */ + else if (x86_64_immediate_operand (src, SImode)) + igain -= COSTS_N_BYTES (2); doesn't it need two GPR xor for 32bit DImode and two mov? Thus the non-SSE cost should be times 'm'? For const0_rtx we may eventually re-use the zero reg for the high part so that is eventually correct. Also I'm missing a 'else' - in the default case there's no cost/benefit of using SSE vs. GPR regs? For SSE it would be a constant pool load. I also wonder, since I now see COSTS_N_BYTES for the first time (heh), whether with -Os we'd need to replace all COSTS_N_INSNS (1) scaling with COSTS_N_BYTES scaling? OTOH costs_add_n_insns uses COSTS_N_INSNS for the size part as well. That said, it looks like we're eventually mixing apples and oranges now or even previously? Thanks, Richard. > > > 2021-08-19 Roger Sayle <ro...@nextmovesoftware.com> > > gcc/ChangeLog > * config/i386/i386-features.c (compute_convert_gain): Provide > more accurate values for CONST_INT, when optimizing for size. > * config/i386/i386.c (COSTS_N_BYTES): Move definition from here... > * config/i386/i386.h (COSTS_N_BYTES): to here. > > Roger > -- >
