Mesa (master): aco: fix a hazard with v_interp_* and v_{read,readfirst}lane_* on GFX6

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Module: Mesa
Branch: master
Commit: 1ac49ba908acf70a8ae4aad71dc715bf625aea1e
URL:    http://cgit.freedesktop.org/mesa/mesa/commit/?id=1ac49ba908acf70a8ae4aad71dc715bf625aea1e

Author: Samuel Pitoiset <samuel.pitoiset at gmail.com>
Date:   Tue Jan 21 16:49:22 2020 +0100

aco: fix a hazard with v_interp_* and v_{read,readfirst}lane_* on GFX6

It's required to insert 1 wait state if the dst VGPR of any v_interp_*
is followed by a read with v_readfirstlane or v_readlane to fix GPU
hangs on GFX6. Note that v_writelane_* is apparently not affected.
This hazard isn't documented anywhere but AMD confirmed it.

This fixes a GPU hang with the texturemipmapgen Sascha demo on GFX6.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset at gmail.com>
Reviewed-by: Daniel Schürmann <daniel at schuermann.dev>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3533>

---

 src/amd/compiler/README.md           |  9 +++++++++
 src/amd/compiler/aco_insert_NOPs.cpp | 18 ++++++++++++++++++
 2 files changed, 27 insertions(+)

diff --git a/src/amd/compiler/README.md b/src/amd/compiler/README.md
index 7c7e68f458e..9c47c07bc80 100644
--- a/src/amd/compiler/README.md
+++ b/src/amd/compiler/README.md
@@ -131,6 +131,15 @@ finish and then write to vcc (for example, `s_mov_b64 vcc, vcc`) to correct vccz
 
 Currently, we don't do this.
 
+## GCN / GFX6 hazards
+
+### VINTRP followed by a read with v_readfirstlane or v_readlane
+
+It's required to insert 1 wait state if the dst VGPR of any v_interp_* is
+followed by a read with v_readfirstlane or v_readlane to fix GPU hangs on GFX6.
+Note that v_writelane_* is apparently not affected. This hazard isn't
+documented anywhere but AMD confirmed it.
+
 ## RDNA / GFX10 hazards
 
 ### SMEM store followed by a load with the same address
diff --git a/src/amd/compiler/aco_insert_NOPs.cpp b/src/amd/compiler/aco_insert_NOPs.cpp
index 689d5e25acc..8c032bb699c 100644
--- a/src/amd/compiler/aco_insert_NOPs.cpp
+++ b/src/amd/compiler/aco_insert_NOPs.cpp
@@ -353,6 +353,24 @@ int handle_instruction_gfx8_9(NOP_ctx_gfx8_9& ctx, aco_ptr<Instruction>& instr,
                ctx.VALU_wrsgpr = NOPs ? new_idx : new_idx + 1;
          }
       }
+
+      /* It's required to insert 1 wait state if the dst VGPR of any v_interp_*
+       * is followed by a read with v_readfirstlane or v_readlane to fix GPU
+       * hangs on GFX6. Note that v_writelane_* is apparently not affected.
+       * This hazard isn't documented anywhere but AMD confirmed that hazard.
+       */
+      if (ctx.chip_class == GFX6 &&
+          !new_instructions.empty() &&
+          (instr->opcode == aco_opcode::v_readfirstlane_b32 ||
+           instr->opcode == aco_opcode::v_readlane_b32)) {
+         aco_ptr<Instruction>& pred = new_instructions.back();
+         if (pred->format == Format::VINTRP) {
+            Definition pred_def = pred->definitions[0];
+            Operand& op = instr->operands[0];
+            if (regs_intersect(pred_def.physReg(), pred_def.size(), op.physReg(), op.size()))
+               NOPs = std::max(NOPs, 1);
+         }
+      }
       return NOPs;
    } else if (instr->isVMEM() && ctx.VALU_wrsgpr + 5 >= new_idx) {
       /* If the VALU writes the SGPR that is used by a VMEM, the user must add five wait states. */