intel/elk: Fix optimize_extract_to_float for i2f of unsigned extract

Fixes fs-uint-to-float-of-extract-int8.shader_test and
fs-uint-to-float-of-extract-int16.shader_test added by piglit!883.

v2: Expand the comment explaining the potential problem. Suggested by
Caio.

Fixes: e6022281f27 ("intel/elk: Rename files to use elk prefix")
Reviewed-by: Caio Oliveira <caio.oliveira@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/27891>
(cherry picked from commit 0fa17962d6b26fe29996a5767fbdd44dc2dbd082)

2 files changed, 39 insertions, 6 deletions

diff --git a/.pick_status.json b/.pick_status.json
index f753a2e315c..aa5632ea223 100644
--- a/.pick_status.json
+++ b/.pick_status.json
@@ -64,7 +64,7 @@
         "description": "intel/elk: Fix optimize_extract_to_float for i2f of unsigned extract",
         "nominated": true,
         "nomination_type": 1,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "e6022281f273287499e9012f9a7c3dd87a733e5b",
         "notes": null
diff --git a/src/intel/compiler/elk/elk_fs_nir.cpp b/src/intel/compiler/elk/elk_fs_nir.cpp
index 75f0e6e610f..799e59a3e4a 100644
--- a/src/intel/compiler/elk/elk_fs_nir.cpp
+++ b/src/intel/compiler/elk/elk_fs_nir.cpp
@@ -479,6 +479,9 @@ optimize_extract_to_float(nir_to_elk_state &ntb, nir_alu_instr *instr,
    const intel_device_info *devinfo = ntb.devinfo;
    const fs_builder &bld = ntb.bld;
 
+   /* No fast path for f16 or f64. */
+   assert(instr->op == nir_op_i2f32 || instr->op == nir_op_u2f32);
+
    if (!instr->src[0].src.ssa->parent_instr)
       return false;
 
@@ -488,16 +491,46 @@ optimize_extract_to_float(nir_to_elk_state &ntb, nir_alu_instr *instr,
    nir_alu_instr *src0 =
       nir_instr_as_alu(instr->src[0].src.ssa->parent_instr);
 
-   if (src0->op != nir_op_extract_u8 && src0->op != nir_op_extract_u16 &&
-       src0->op != nir_op_extract_i8 && src0->op != nir_op_extract_i16)
+   unsigned bytes;
+   bool is_signed;
+
+   switch (src0->op) {
+   case nir_op_extract_u8:
+   case nir_op_extract_u16:
+      bytes = src0->op == nir_op_extract_u8 ? 1 : 2;
+
+      /* i2f(extract_u8(a, b)) and u2f(extract_u8(a, b)) produce the same
+       * result. Ditto for extract_u16.
+       */
+      is_signed = false;
+      break;
+
+   case nir_op_extract_i8:
+   case nir_op_extract_i16:
+      bytes = src0->op == nir_op_extract_i8 ? 1 : 2;
+
+      /* The fast path can't handle u2f(extract_i8(a, b)) because the implicit
+       * sign extension of the extract_i8 is lost. For example,
+       * u2f(extract_i8(0x0000ff00, 1)) should produce 4294967295.0, but a
+       * fast path could either give 255.0 (by implementing the fast path as
+       * u2f(extract_u8(x))) or -1.0 (by implementing the fast path as
+       * i2f(extract_i8(x))). At one point in time, we incorrectly implemented
+       * the former.
+       */
+      if (instr->op != nir_op_i2f32)
+         return false;
+
+      is_signed = true;
+      break;
+
+   default:
       return false;
+   }
 
    unsigned element = nir_src_as_uint(src0->src[1].src);
 
    /* Element type to extract.*/
-   const elk_reg_type type = elk_int_type(
-      src0->op == nir_op_extract_u16 || src0->op == nir_op_extract_i16 ? 2 : 1,
-      src0->op == nir_op_extract_i16 || src0->op == nir_op_extract_i8);
+   const elk_reg_type type = elk_int_type(bytes, is_signed);
 
    elk_fs_reg op0 = get_nir_src(ntb, src0->src[0].src);
    op0.type = elk_type_for_nir_type(devinfo,