brw: Don't mark_invalid in update_for_reads for non-VGRF destination

This can occur if NULL or an accumulator is an explicit destination.
update_for_reads still needs to process the sources.

v2: Pass a brw_reg to ::mark_invalid, and do the VGRF check in that one
place.

Fixes: 0d144821f0 ("intel/brw: Add a new def analysis pass")
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
(cherry picked from commit a548466186)

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/40359>

.pick_status.json

@@ -2964,7 +2964,7 @@
         "description": "brw: Don't mark_invalid in update_for_reads for non-VGRF destination",
         "nominated": true,
         "nomination_type": 2,
-        "resolution": 0,
+        "resolution": 1,
         "main_sha": null,
         "because_sha": "0d144821f0f7647ab377e629599b17dd24a2fe13",
         "notes": null

src/intel/compiler/brw/brw_analysis.h

@@ -396,7 +396,7 @@ public:
    bool validate(const brw_shader *) const;
 
 private:
-   void mark_invalid(int);
+   void mark_invalid(const brw_reg &);
    bool fully_defines(const brw_shader *v, brw_inst *);
    void update_for_reads(const brw_idom_tree &idom, brw_inst *);
    void update_for_write(const brw_shader *v, brw_inst *);

src/intel/compiler/brw/brw_analysis_def.cpp

@@ -37,9 +37,10 @@
 static brw_inst *const UNSEEN = (brw_inst *) (uintptr_t) 1;
 
 void
-brw_def_analysis::mark_invalid(int nr)
+brw_def_analysis::mark_invalid(const brw_reg &reg)
 {
-   def_insts[nr] = NULL;
+   if (reg.file == VGRF)
+      def_insts[reg.nr] = NULL;
 }
 
 void
@@ -50,7 +51,7 @@ brw_def_analysis::update_for_reads(const brw_idom_tree &idom,
     * implicitly reads the accumulator, we don't consider it to produce a def.
     */
    if (inst->reads_accumulator_implicitly())
-      mark_invalid(inst->dst.nr);
+      mark_invalid(inst->dst);
 
    for (int i = 0; i < inst->sources; i++) {
       const int nr = inst->src[i].nr;
@@ -63,7 +64,7 @@ brw_def_analysis::update_for_reads(const brw_idom_tree &idom,
              (nr == BRW_ARF_ADDRESS ||
               nr == BRW_ARF_ACCUMULATOR ||
               nr == BRW_ARF_FLAG))
-            mark_invalid(inst->dst.nr);
+            mark_invalid(inst->dst);
 
          continue;
       }
@@ -79,7 +80,7 @@ brw_def_analysis::update_for_reads(const brw_idom_tree &idom,
           */
          if (def_insts[nr] == UNSEEN ||
              !idom.dominates(def_insts[nr]->block, inst->block))
-            mark_invalid(nr);
+            mark_invalid(inst->src[i]);
       }
 
       /* Additionally, if one of our sources is not a def, then our
@@ -87,8 +88,8 @@ brw_def_analysis::update_for_reads(const brw_idom_tree &idom,
        */
       if (inst->opcode != SHADER_OPCODE_LOAD_REG &&
           inst->opcode != SHADER_OPCODE_LOAD_PAYLOAD &&
-          !def_insts[nr] && inst->dst.file == VGRF)
-         mark_invalid(inst->dst.nr);
+          !def_insts[nr])
+         mark_invalid(inst->dst);
    }
 }
 
@@ -117,7 +118,7 @@ brw_def_analysis::update_for_write(const brw_shader *v,
       /* Otherwise this is a second write or a partial write, in which
        * case we know with certainty that this isn't an SSA def.
        */
-      mark_invalid(nr);
+      mark_invalid(inst->dst);
    }
 }
 
@@ -162,7 +163,7 @@ brw_def_analysis::brw_def_analysis(const brw_shader *v)
             /* If our "def" reads a non-SSA source, then it isn't a def. */
             if (def->opcode != SHADER_OPCODE_LOAD_REG &&
                 (!def_insts[nr] || def_insts[nr] == UNSEEN)) {
-               mark_invalid(def->dst.nr);
+               mark_invalid(def->dst);
                iterate = true;
                break;
             }

src/intel/compiler/brw/meson.build

@@ -159,6 +159,7 @@ if with_tests
     executable(
       'intel_compiler_brw_tests',
       files(
+        'test_def_analysis.cpp',
         'test_eu_compact.cpp',
         'test_eu_validate.cpp',
         'test_helpers.cpp',

src/intel/compiler/brw/test_def_analysis.cpp

@@ -0,0 +1,69 @@
+/*
+ * Copyright 2026 Intel Corporation
+ * SPDX-License-Identifier: MIT
+ */
+
+#include "test_helpers.h"
+#include "brw_builder.h"
+
+class defs_test : public brw_shader_pass_test {};
+
+TEST_F(defs_test, dst_is_acc0)
+{
+   set_gfx_verx10(125);
+
+   brw_builder bld = make_shader(MESA_SHADER_FRAGMENT, 16);
+   brw_reg acc0 = retype(brw_acc_reg(16), BRW_TYPE_F);
+   brw_reg dst0;
+
+   for (unsigned i = 0; i < 1024; i++) {
+      dst0 = vgrf(bld, BRW_TYPE_F);
+
+      if (dst0.nr == acc0.nr)
+         break;
+   }
+
+   ASSERT_EQ(dst0.nr, acc0.nr);
+
+   brw_reg src0 = vgrf(bld, BRW_TYPE_F);
+
+   brw_inst *inst = bld.MOV(dst0, brw_imm_f(1.0));
+   bld.MOV(src0, brw_imm_f(2.0));
+   bld.MAC(acc0, src0, brw_imm_f(3.0));
+
+   brw_calculate_cfg(*bld.shader);
+   brw_validate(*bld.shader);
+
+   const brw_def_analysis &defs = bld.shader->def_analysis.require();
+
+   EXPECT_EQ(inst, defs.get(dst0));
+}
+
+TEST_F(defs_test, dst_and_src_are_acc0)
+{
+   set_gfx_verx10(125);
+
+   brw_builder bld = make_shader(MESA_SHADER_FRAGMENT, 16);
+   brw_reg acc0 = retype(brw_acc_reg(16), BRW_TYPE_F);
+   brw_reg dst0;
+
+   for (unsigned i = 0; i < 1024; i++) {
+      dst0 = vgrf(bld, BRW_TYPE_F);
+
+      if (dst0.nr == acc0.nr)
+         break;
+   }
+
+   ASSERT_EQ(dst0.nr, acc0.nr);
+
+   brw_inst *inst = bld.MOV(dst0, brw_imm_f(1.0));
+   bld.MOV(acc0, brw_imm_f(2.0));
+   bld.MUL(acc0, acc0, brw_imm_f(3.0));
+
+   brw_calculate_cfg(*bld.shader);
+   brw_validate(*bld.shader);
+
+   const brw_def_analysis &defs = bld.shader->def_analysis.require();
+
+   EXPECT_EQ(inst, defs.get(dst0));
+}