[Mesa-dev] [PATCH v3 4/4] nv50/ir: further optimize multiplication by immediates
Rhys Perry
pendingchaos02 at gmail.com
Sat Aug 11 14:40:18 UTC 2018
It seems multiplication by negative powers of two are nonexistent in the
shader-db, so an specialized optimization for them would probably not be
worth it.
It seems my approach gives better instruction counts in shader-db than
your approach, since it can generate shorter (for things like a * 7) and
more LoadPropagation/ModifierPropagation friendly code.
Yours gives slightly better gpr counts overall, though a decent bit of
shaders seem to do much better gpr-wise with my approach. It also seems to
end up being slightly worse gpr-wise after making the instruction count
closer to my approach.
Comparing my approach to yours:
total instructions in shared programs : 5802201 -> 5818813 (0.29%)
total gprs used in shared programs : 669876 -> 669708 (-0.03%)
total shared used in shared programs : 548832 -> 548832 (0.00%)
total local used in shared programs : 21068 -> 21068 (0.00%)
local shared gpr inst bytes
helped 0 0 322 105 105
hurt 0 0 133 2711 2711
Comparing my approach to yours after making it closer to mine by adding a
few more specializations:
total instructions in shared programs : 5802201 -> 5807994 (0.10%)
total gprs used in shared programs : 669876 -> 669987 (0.02%)
total shared used in shared programs : 548832 -> 548832 (0.00%)
total local used in shared programs : 21068 -> 21068 (0.00%)
local shared gpr inst bytes
helped 0 0 142 416 416
hurt 0 0 109 489 489
On Sat, Aug 11, 2018 at 11:37 AM Karol Herbst <kherbst at redhat.com> wrote:
>
> I think we could do something else (which may even cover more cases):
>
> 1. try to use a shl (we already do that)
>
> 2 use shladd for all negative imms with for all power of two negative
> immediates (are we already doing it? I think we miss a lot of opts
> where "worse" instructions could include modifiers and we basically
> save neg/abs instructions. OP_SHL can't take a neg, so we have to use
> OP_SHLADD for that)
> b = shladd(neg a, log2(imm), 0x0)
>
> 3. for all immediates in [0xffff, 0x0]:
> t = xmad(a, imm, 0x0)
> b = xmad.PSL(a.hi, imm, t)
>
> which should be already quite good.
>
> I don't know if using shifts or shladd is faster then xmad, so without
> benchmarks I wouldn't want to include more complex optimizations if we
> don't know they pay off. Nvidia doesn't seem to do that either, but
> they use shladd in that negative immediate case. Maybe they don't do
> it for the trivial case shladd(a, log2(imm), a) for power of two + 1
> because they simply don't care. Don't know. I am sure it would be
> worth it to see where it actually makes a different adding your opts
> after the three above ones. Or maybe yours is also faster than the
> third one above. I don't know.
>
> On Mon, Jul 23, 2018 at 12:40 PM, Rhys Perry <pendingchaos02 at gmail.com> wrote:
> > Strongly mitigates the harm from the previous commit, which made many
> > integer multiplications much more heavy on the register and instruction
> > count.
> >
> > total instructions in shared programs : 5839715 -> 5801926 (-0.65%)
> > total gprs used in shared programs : 670553 -> 669853 (-0.10%)
> > total shared used in shared programs : 548832 -> 548832 (0.00%)
> > total local used in shared programs : 21164 -> 21068 (-0.45%)
> >
> > local shared gpr inst bytes
> > helped 1 0 408 2522 2522
> > hurt 0 0 232 23 23
> >
> > Signed-off-by: Rhys Perry <pendingchaos02 at gmail.com>
> > ---
> > .../drivers/nouveau/codegen/nv50_ir_peephole.cpp | 126 ++++++++++++++++++---
> > 1 file changed, 112 insertions(+), 14 deletions(-)
> >
> > diff --git a/src/gallium/drivers/nouveau/codegen/nv50_ir_peephole.cpp b/src/gallium/drivers/nouveau/codegen/nv50_ir_peephole.cpp
> > index a6ddb284b8..e5d033c9b0 100644
> > --- a/src/gallium/drivers/nouveau/codegen/nv50_ir_peephole.cpp
> > +++ b/src/gallium/drivers/nouveau/codegen/nv50_ir_peephole.cpp
> > @@ -379,6 +379,10 @@ private:
> >
> > CmpInstruction *findOriginForTestWithZero(Value *);
> >
> > + Value *createMulMethod1(Value *a, unsigned b, Value *c);
> > + Value *createMulMethod2(Value *a, unsigned b, Value *c);
> > + Value *createMul(Value *a, unsigned b, Value *c);
> > +
> > unsigned int foldCount;
> >
> > BuildUtil bld;
> > @@ -953,6 +957,88 @@ ConstantFolding::opnd3(Instruction *i, ImmediateValue &imm2)
> > }
> > }
> >
> > +Value *
> > +ConstantFolding::createMulMethod1(Value *a, unsigned b, Value *c)
> > +{
> > + // basically constant folded shift and add multiplication.
> > + Value *res = c ? c : bld.loadImm(NULL, 0u);
> > + bool resZero = !c;
> > + unsigned ashift = 0;
> > + while (b) {
> > + if ((b & 1) && ashift) {
> > + Value *sh = bld.loadImm(NULL, ashift);
> > + if (resZero)
> > + res = bld.mkOp2v(OP_SHL, TYPE_U32, bld.getSSA(), a, sh);
> > + else
> > + res = bld.mkOp3v(OP_SHLADD, TYPE_U32, bld.getSSA(), a, sh, res);
> > + resZero = false;
> > + } else if (b & 1) {
> > + if (resZero)
> > + res = a;
> > + else
> > + res = bld.mkOp2v(OP_ADD, TYPE_U32, bld.getSSA(), res, a);
> > + resZero = false;
> > + }
> > + b >>= 1;
> > + ashift++;
> > + }
> > + return res;
> > +}
> > +
> > +Value *
> > +ConstantFolding::createMulMethod2(Value *a, unsigned b, Value *c)
> > +{
> > + // basically does a * b2 - a * (b2 - b) + c
> > + uint64_t b2 = util_next_power_of_two64(b);
> > + unsigned b2shift = ffsll(b2) - 1;
> > +
> > + Value *mul1 = createMulMethod1(a, b2 - b, NULL);
> > +
> > + Value *res;
> > + if (b2shift < 32) {
> > + Instruction *i = bld.mkOp3(OP_SHLADD, TYPE_U32, bld.getSSA(),
> > + a, bld.loadImm(NULL, b2shift), mul1);
> > + res = i->getDef(0);
> > +
> > + // all targets supporting OP_SHLADD should pass this
> > + assert(bld.getProgram()->getTarget()->isModSupported(i, 2, NV50_IR_MOD_NEG));
> > + i->src(2).mod *= Modifier(NV50_IR_MOD_NEG);
> > + } else {
> > + res = bld.mkOp1v(OP_NEG, TYPE_U32, bld.getSSA(), mul1);
> > + }
> > +
> > + if (c)
> > + res = bld.mkOp2v(OP_ADD, TYPE_U32, bld.getSSA(), res, c);
> > +
> > + return res;
> > +}
> > +
> > +Value *
> > +ConstantFolding::createMul(Value *a, unsigned b, Value *c)
> > +{
> > + unsigned cost[2];
> > +
> > + // estimate cost for first method (a << i) + (b << j) + ...
> > + cost[0] = util_bitcount64(b >> 1);
> > +
> > + // estimate cost for second method (a << i) - ((a << j) + (a << k) + ...)
> > + uint64_t rounded_b = util_next_power_of_two64(b);
> > + cost[1] = rounded_b == b ? 1 : (util_bitcount64((rounded_b - b) >> 1) + 1);
> > + if (c) cost[1]++;
> > +
> > + // The general method, multiplication by XMADs, costs three instructions.
> > + // So nothing much larger than that or it could be making things worse.
> > + if (cost[0] > 3 && cost[1] > 3)
> > + return NULL;
> > +
> > + // the cost is the same for both methods with powers of twos
> > + // but method 1 creates more optimizable code
> > + if (cost[0] <= cost[1])
> > + return createMulMethod1(a, b, c);
> > + else
> > + return createMulMethod2(a, b, c);
> > +}
> > +
> > void
> > ConstantFolding::opnd(Instruction *i, ImmediateValue &imm0, int s)
> > {
> > @@ -1040,13 +1126,25 @@ ConstantFolding::opnd(Instruction *i, ImmediateValue &imm0, int s)
> > i->setSrc(s, i->getSrc(t));
> > i->src(s).mod = i->src(t).mod;
> > } else
> > - if (!isFloatType(i->sType) && !imm0.isNegative() && imm0.isPow2()) {
> > - i->op = OP_SHL;
> > - imm0.applyLog2();
> > - i->setSrc(0, i->getSrc(t));
> > - i->src(0).mod = i->src(t).mod;
> > - i->setSrc(1, new_ImmediateValue(prog, imm0.reg.data.u32));
> > - i->src(1).mod = 0;
> > + if (!isFloatType(i->dType)) {
> > + bool optimized = false;
> > + if (target->isOpSupported(OP_SHLADD, TYPE_U32)) {
> > + bld.setPosition(i, false);
> > + Value *val = createMul(i->getSrc(t), imm0.reg.data.u32, NULL);
> > + if (val) {
> > + i->def(0).replace(val, false);
> > + delete_Instruction(prog, i);
> > + optimized = true;
> > + }
> > + }
> > + if (!optimized && !imm0.isNegative() && imm0.isPow2()) {
> > + i->op = OP_SHL;
> > + imm0.applyLog2();
> > + i->setSrc(0, i->getSrc(t));
> > + i->src(0).mod = i->src(t).mod;
> > + i->setSrc(1, new_ImmediateValue(prog, imm0.reg.data.u32));
> > + i->src(1).mod = 0;
> > + }
> > } else
> > if (i->postFactor && i->sType == TYPE_F32) {
> > /* Can't emit a postfactor with an immediate, have to fold it in */
> > @@ -1079,13 +1177,13 @@ ConstantFolding::opnd(Instruction *i, ImmediateValue &imm0, int s)
> > i->setSrc(2, NULL);
> > i->op = OP_ADD;
> > } else
> > - if (s == 1 && !imm0.isNegative() && imm0.isPow2() &&
> > - !isFloatType(i->dType) &&
> > - target->isOpSupported(OP_SHLADD, i->dType) &&
> > - !i->subOp) {
> > - i->op = OP_SHLADD;
> > - imm0.applyLog2();
> > - i->setSrc(1, new_ImmediateValue(prog, imm0.reg.data.u32));
> > + if (!isFloatType(i->dType) && target->isOpSupported(OP_SHLADD, TYPE_U32) && !i->subOp) {
> > + bld.setPosition(i, false);
> > + Value *val = createMul(i->getSrc(t), imm0.reg.data.u32, i->getSrc(2));
> > + if (val) {
> > + i->def(0).replace(val, false);
> > + delete_Instruction(prog, i);
> > + }
> > }
> > break;
> > case OP_SUB:
> > --
> > 2.14.4
> >
> > _______________________________________________
> > mesa-dev mailing list
> > mesa-dev at lists.freedesktop.org
> > https://lists.freedesktop.org/mailman/listinfo/mesa-dev
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