[Mesa-dev] [PATCH 02/12] i965/fs: Rewrite fsign64 to skip the float -> double conversion

[Matt Turner]

... without the float -> double conversion. Low power parts have
additional restrictions when it comes to operating on 64-bit types, and
the instruction used to do the conversion violates one of them:
specifically, the restriction that "Source and Destination horizontal
stride must be aligned to the same qword".

Previously we generated a float and then converted, but we can avoid the
conversion by using the same extract-the-sign-bit + or-in-1.0 algorithm
by directly operating on the high four bytes of each double-precision
component in the result.

In SIMD8 and SIMD16 this cuts one instruction from the implementation,
and more importantly that instruction is the one which violated the
regioning restriction.

Along the way I removed some comments that I did not think helped, and
some code about double comparisons which does not seem to be necessary
today.

This prevents validation failures caught by the new EU validation code
added in later patches.
---
 src/intel/compiler/brw_fs_nir.cpp | 49 +++++++--------------------------------
 1 file changed, 9 insertions(+), 40 deletions(-)

diff --git a/src/intel/compiler/brw_fs_nir.cpp b/src/intel/compiler/brw_fs_nir.cpp
index 8ea4297ceb..1b011172a6 100644
--- a/src/intel/compiler/brw_fs_nir.cpp
+++ b/src/intel/compiler/brw_fs_nir.cpp
@@ -697,49 +697,18 @@ fs_visitor::nir_emit_alu(const fs_builder &bld, nir_alu_instr *instr)
           * - We need to produce a DF result.
           */
 
-         /* 2-src instructions can't have 64-bit immediates, so put 0.0 in
-          * a register and compare with that.
-          */
-         fs_reg tmp = vgrf(glsl_type::double_type);
-         bld.MOV(tmp, setup_imm_df(bld, 0.0));
-
-         /* A direct DF CMP using the flag register (null dst) won't work in
-          * SIMD16 because the CMP will be split in two by lower_simd_width,
-          * resulting in two CMP instructions with the same dst (NULL),
-          * leading to dead code elimination of the first one. In SIMD8,
-          * however, there is no need to split the CMP and we can save some
-          * work.
-          */
-         fs_reg dst_tmp = vgrf(glsl_type::double_type);
-         bld.CMP(dst_tmp, op[0], tmp, BRW_CONDITIONAL_NZ);
-
-         /* In SIMD16 we want to avoid using a NULL dst register with DF CMP,
-          * so we store the result of the comparison in a vgrf instead and
-          * then we generate a UD comparison from that that won't have to
-          * be split by lower_simd_width. This is what NIR does to handle
-          * double comparisons in the general case.
-          */
-         if (bld.dispatch_width() == 16 ) {
-            fs_reg dst_tmp_ud = retype(dst_tmp, BRW_REGISTER_TYPE_UD);
-            bld.MOV(dst_tmp_ud, subscript(dst_tmp, BRW_REGISTER_TYPE_UD, 0));
-            bld.CMP(bld.null_reg_ud(),
-                    dst_tmp_ud, brw_imm_ud(0), BRW_CONDITIONAL_NZ);
-         }
-
-         /* Get the high 32-bit of each double component where the sign is */
-         fs_reg result_int = retype(result, BRW_REGISTER_TYPE_UD);
-         bld.MOV(result_int, subscript(op[0], BRW_REGISTER_TYPE_UD, 1));
+         fs_reg zero = vgrf(glsl_type::double_type);
+         bld.MOV(zero, setup_imm_df(bld, 0.0));
+         bld.CMP(bld.null_reg_df(), op[0], zero, BRW_CONDITIONAL_NZ);
 
-         /* Get the sign bit */
-         bld.AND(result_int, result_int, brw_imm_ud(0x80000000u));
+         bld.MOV(result, zero);
 
-         /* Add 1.0 to the sign, predicated to skip the case of op[0] == 0.0 */
-         inst = bld.OR(result_int, result_int, brw_imm_ud(0x3f800000u));
-         inst->predicate = BRW_PREDICATE_NORMAL;
+         fs_reg r = subscript(result, BRW_REGISTER_TYPE_UD, 1);
+         bld.AND(r, subscript(op[0], BRW_REGISTER_TYPE_UD, 1),
+                 brw_imm_ud(0x80000000u));
 
-         /* Convert from 32-bit float to 64-bit double */
-         result.type = BRW_REGISTER_TYPE_DF;
-         inst = bld.MOV(result, retype(result_int, BRW_REGISTER_TYPE_F));
+         set_predicate(BRW_PREDICATE_NORMAL,
+                       bld.OR(r, r, brw_imm_ud(0x3ff00000u)));
 
          if (instr->dest.saturate) {
             inst = bld.MOV(result, result);
-- 
2.13.6