[Mesa-dev] [PATCH 16/24] i965: Use LZD to implement nir_op_ifind_msb on Gen < 7
Francisco Jerez
currojerez at riseup.net
Thu Jun 30 23:55:38 UTC 2016
Ian Romanick <idr at freedesktop.org> writes:
> From: Ian Romanick <ian.d.romanick at intel.com>
>
> Signed-off-by: Ian Romanick <ian.d.romanick at intel.com>
> ---
> src/mesa/drivers/dri/i965/brw_fs_nir.cpp | 50 ++++++++++++++++++++++------
> src/mesa/drivers/dri/i965/brw_vec4_nir.cpp | 52 +++++++++++++++++++++++-------
> 2 files changed, 81 insertions(+), 21 deletions(-)
>
> diff --git a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
> index f15bf3e..f8db28a 100644
> --- a/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_fs_nir.cpp
> @@ -623,8 +623,32 @@ fs_visitor::nir_emit_find_msb_using_lzd(const fs_builder &bld,
> bool is_signed)
> {
> fs_inst *inst;
> + fs_reg temp = src;
>
> - bld.LZD(retype(result, BRW_REGISTER_TYPE_UD), src);
> + if (is_signed) {
> + /* LZD of an absolute value source almost always does the right
> + * thing. There are two problem values:
> + *
This comment seems somewhat misleading, there are many more problem
values if you attempt to just take the LZD of the absolute value of the
argument: Take the negative of any power of two e.g. -8 represented as
0xfffffff8. findMSB(-8) is supposed to give you 2 as result, but '31
- lzd(abs(-8)) == 31 - 28 == 3'.
> + * * 0x80000000. Since abs(0x80000000) == 0x80000000, LZD returns
> + * 0. However, findMSB(int(0x80000000)) == 30.
> + *
> + * * 0xffffffff. Since abs(0xffffffff) == 1, LZD returns
> + * 31. Section 8.8 (Integer Functions) of the GLSL 4.50 spec says:
> + *
> + * For a value of zero or negative one, -1 will be returned.
> + *
> + * For all negative number cases, including 0x80000000 and
> + * 0xffffffff, the correct value is obtained from LZD if instead of
> + * negating the (already negative) value the logical-not is used. A
> + * conditonal logical-not can be achieved in two instructions.
> + */
> + temp = vgrf(glsl_type::int_type);
> +
> + bld.ASR(temp, src, brw_imm_d(31));
> + bld.XOR(temp, temp, src);
> + }
> +
> + bld.LZD(retype(result, BRW_REGISTER_TYPE_UD), temp);
>
> /* LZD counts from the MSB side, while GLSL's findMSB() wants the count
> * from the LSB side. Subtract the result from 31 to convert the MSB
> @@ -1339,17 +1363,23 @@ fs_visitor::nir_emit_alu(const fs_builder &bld, nir_alu_instr *instr)
>
> case nir_op_ifind_msb: {
> assert(nir_dest_bit_size(instr->dest.dest) < 64);
> - bld.FBH(retype(result, BRW_REGISTER_TYPE_UD), op[0]);
>
> - /* FBH counts from the MSB side, while GLSL's findMSB() wants the count
> - * from the LSB side. If FBH didn't return an error (0xFFFFFFFF), then
> - * subtract the result from 31 to convert the MSB count into an LSB count.
> - */
> - bld.CMP(bld.null_reg_d(), result, brw_imm_d(-1), BRW_CONDITIONAL_NZ);
> + if (devinfo->gen < 7) {
> + nir_emit_find_msb_using_lzd(bld, result, op[0], true);
> + } else {
> + bld.FBH(retype(result, BRW_REGISTER_TYPE_UD), op[0]);
>
> - inst = bld.ADD(result, result, brw_imm_d(31));
> - inst->predicate = BRW_PREDICATE_NORMAL;
> - inst->src[0].negate = true;
> + /* FBH counts from the MSB side, while GLSL's findMSB() wants the
> + * count from the LSB side. If FBH didn't return an error
> + * (0xFFFFFFFF), then subtract the result from 31 to convert the MSB
> + * count into an LSB count.
> + */
> + bld.CMP(bld.null_reg_d(), result, brw_imm_d(-1), BRW_CONDITIONAL_NZ);
> +
> + inst = bld.ADD(result, result, brw_imm_d(31));
> + inst->predicate = BRW_PREDICATE_NORMAL;
> + inst->src[0].negate = true;
> + }
> break;
> }
>
> diff --git a/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp b/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
> index cd88fb6..2fc2cf2 100644
> --- a/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
> +++ b/src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
> @@ -999,8 +999,32 @@ vec4_visitor::nir_emit_find_msb_using_lzd(const dst_reg &dst,
> bool is_signed)
> {
> vec4_instruction *inst;
> + src_reg temp = src;
>
> - emit(BRW_OPCODE_LZD, retype(dst, BRW_REGISTER_TYPE_UD), src);
> + if (is_signed) {
> + /* LZD of an absolute value source almost always does the right
> + * thing. There are two problem values:
> + *
> + * * 0x80000000. Since abs(0x80000000) == 0x80000000, LZD returns
> + * 0. However, findMSB(int(0x80000000)) == 30.
> + *
> + * * 0xffffffff. Since abs(0xffffffff) == 1, LZD returns
> + * 31. Section 8.8 (Integer Functions) of the GLSL 4.50 spec says:
> + *
> + * For a value of zero or negative one, -1 will be returned.
> + *
> + * For all negative number cases, including 0x80000000 and
> + * 0xffffffff, the correct value is obtained from LZD if instead of
> + * negating the (already negative) value the logical-not is used. A
> + * conditonal logical-not can be achieved in two instructions.
> + */
> + temp = src_reg(this, glsl_type::ivec4_type);
> +
> + emit(BRW_OPCODE_ASR, dst_reg(temp), src, brw_imm_d(31));
> + emit(BRW_OPCODE_XOR, dst_reg(temp), temp, src);
> + }
> +
> + emit(BRW_OPCODE_LZD, retype(dst, BRW_REGISTER_TYPE_UD), temp);
>
> /* LZD counts from the MSB side, while GLSL's findMSB() wants the count
> * from the LSB side. Subtract the result from 31 to convert the MSB count
> @@ -1484,18 +1508,24 @@ vec4_visitor::nir_emit_alu(nir_alu_instr *instr)
> break;
>
> case nir_op_ifind_msb: {
> - emit(FBH(retype(dst, BRW_REGISTER_TYPE_UD), op[0]));
> -
> - /* FBH counts from the MSB side, while GLSL's findMSB() wants the count
> - * from the LSB side. If FBH didn't return an error (0xFFFFFFFF), then
> - * subtract the result from 31 to convert the MSB count into an LSB count.
> - */
> src_reg src(dst);
> - emit(CMP(dst_null_d(), src, brw_imm_d(-1), BRW_CONDITIONAL_NZ));
>
> - inst = emit(ADD(dst, src, brw_imm_d(31)));
> - inst->predicate = BRW_PREDICATE_NORMAL;
> - inst->src[0].negate = true;
> + if (devinfo->gen < 7) {
> + nir_emit_find_msb_using_lzd(dst, op[0], true);
> + } else {
> + emit(FBH(retype(dst, BRW_REGISTER_TYPE_UD), op[0]));
> +
> + /* FBH counts from the MSB side, while GLSL's findMSB() wants the
> + * count from the LSB side. If FBH didn't return an error
> + * (0xFFFFFFFF), then subtract the result from 31 to convert the MSB
> + * count into an LSB count.
> + */
> + emit(CMP(dst_null_d(), src, brw_imm_d(-1), BRW_CONDITIONAL_NZ));
> +
> + inst = emit(ADD(dst, src, brw_imm_d(31)));
> + inst->predicate = BRW_PREDICATE_NORMAL;
> + inst->src[0].negate = true;
> + }
> break;
> }
>
> --
> 2.5.5
>
> _______________________________________________
> mesa-dev mailing list
> mesa-dev at lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/mesa-dev
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