Ping^3. On Sun, 5 Jan 2020, Alexander Monakov wrote:
> Hi, > > I noticed there's a costly signed 64-bit division in rtx_cost on x86 as well > as > any other target where UNITS_PER_WORD is implemented like TARGET_64BIT ? 8 : > 4. > It's also evident that rtx_cost does redundant work for a SET rtx argument. > > Obviously the variable named 'factor' rarely exceeds 1, so in the majority of > cases it can be computed with a well-predictable branch rather than a > division. > > This patch makes rtx_cost do the division only in case mode is wider than > UNITS_PER_WORD, and also moves a test for a SET up front to avoid redundancy. > No functional change. > > Bootstrapped on x86_64, ok for trunk? > > To illustrate the improvement this buys, for tramp3d -O2 compilation, I got > > before: > 73887675319 instructions:u > > 72438432200 cycles:u > 924298569 idq.ms_uops:u > 102603799255 uops_dispatched.thread:u > > after: > 73888371724 instructions:u > > 72386986612 cycles:u > 802744775 idq.ms_uops:u > 102096987220 uops_dispatched.thread:u > > (this is on Sandybridge, idq.ms_uops are uops going via the microcode > sequencer, > so the unneeded division is responsible for a good fraction of them) > > * rtlanal.c (rtx_cost): Handle a SET up front. Avoid division if the > mode is not wider than UNITS_PER_WORD. > > diff --git a/gcc/rtlanal.c b/gcc/rtlanal.c > index 9a7afccefb8..c7ab86e228b 100644 > --- a/gcc/rtlanal.c > +++ b/gcc/rtlanal.c > @@ -4207,18 +4207,23 @@ rtx_cost (rtx x, machine_mode mode, enum rtx_code > outer_code, > const char *fmt; > int total; > int factor; > + unsigned mode_size; > > if (x == 0) > return 0; > > - if (GET_MODE (x) != VOIDmode) > + if (GET_CODE (x) == SET) > + /* A SET doesn't have a mode, so let's look at the SET_DEST to get > + the mode for the factor. */ > + mode = GET_MODE (SET_DEST (x)); > + else if (GET_MODE (x) != VOIDmode) > mode = GET_MODE (x); > > + mode_size = estimated_poly_value (GET_MODE_SIZE (mode)); > + > /* A size N times larger than UNITS_PER_WORD likely needs N times as > many insns, taking N times as long. */ > - factor = estimated_poly_value (GET_MODE_SIZE (mode)) / UNITS_PER_WORD; > - if (factor == 0) > - factor = 1; > + factor = mode_size > UNITS_PER_WORD ? mode_size / UNITS_PER_WORD : 1; > > /* Compute the default costs of certain things. > Note that targetm.rtx_costs can override the defaults. */ > @@ -4243,14 +4248,6 @@ rtx_cost (rtx x, machine_mode mode, enum rtx_code > outer_code, > /* Used in combine.c as a marker. */ > total = 0; > break; > - case SET: > - /* A SET doesn't have a mode, so let's look at the SET_DEST to get > - the mode for the factor. */ > - mode = GET_MODE (SET_DEST (x)); > - factor = estimated_poly_value (GET_MODE_SIZE (mode)) / UNITS_PER_WORD; > - if (factor == 0) > - factor = 1; > - /* FALLTHRU */ > default: > total = factor * COSTS_N_INSNS (1); > } >