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i965: Use LZD to implement nir_op_ifind_msb on Gen < 7

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v2: Retype LZD source as UD to avoid potential problems with 0x80000000.
Suggested by Matt.  Also update comment about problem values with
LZD(abs(x)).  Suggested by Curro.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
This commit is contained in:
Ian Romanick
2016-06-21 17:18:04 -07:00
parent de20086eed
commit c2019c6c26
2 changed files with 90 additions and 21 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/mesa/drivers/dri/i965/brw_fs_nir.cpp
+44 -10
View File
@@ -621,8 +621,36 @@ 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:
*
* * 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.
*
* * Negative powers of two. LZD(abs(-(1<<x))) returns x, but
* findMSB(-(1<<x)) should return x-1.
*
* 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 = bld.vgrf(BRW_REGISTER_TYPE_D);
bld.ASR(temp, src, brw_imm_d(31));
bld.XOR(temp, temp, src);
}
bld.LZD(retype(result, BRW_REGISTER_TYPE_UD),
retype(temp, BRW_REGISTER_TYPE_UD));
/* 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
@@ -1337,17 +1365,23 @@ 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) {
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;
}
src/mesa/drivers/dri/i965/brw_vec4_nir.cpp
+46 -11
View File
@@ -1000,8 +1000,36 @@ emit_find_msb_using_lzd(const vec4_builder &bld,
bool is_signed)
{
vec4_instruction *inst;
src_reg temp = src;
bld.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.
*
* * Negative powers of two. LZD(abs(-(1<<x))) returns x, but
* findMSB(-(1<<x)) should return x-1.
*
* 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(bld.vgrf(BRW_REGISTER_TYPE_D));
bld.ASR(dst_reg(temp), src, brw_imm_d(31));
bld.XOR(dst_reg(temp), temp, src);
}
bld.LZD(retype(dst, BRW_REGISTER_TYPE_UD),
retype(temp, BRW_REGISTER_TYPE_UD));
/* 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
@@ -1485,18 +1513,25 @@ 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.
*/
vec4_builder bld = vec4_builder(this).at_end();
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) {
emit_find_msb_using_lzd(bld, 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.
*/
bld.CMP(dst_null_d(), src, brw_imm_d(-1), BRW_CONDITIONAL_NZ);
inst = bld.ADD(dst, src, brw_imm_d(31));
inst->predicate = BRW_PREDICATE_NORMAL;
inst->src[0].negate = true;
}
break;
}