From bb956464cbc3d5012527ea3d16dbd3f2c96cdae1 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timur=20Krist=C3=B3f?= <timur.kristof@gmail.com>
Date: Wed, 1 Sep 2021 18:09:01 +0200
Subject: [PATCH] aco: Skip code paths to emit copies when there are no copies.
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Found while running with libstdc++ debug mode.
Fixes the following:

Error: attempt to advance a dereferenceable (start-of-sequence)
iterator -1 steps, which falls outside its valid range.

Cc: mesa-stable
Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Reviewed-by: Tony Wasserka <tony.wasserka@gmx.de>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12682>
---
 src/amd/compiler/aco_lower_to_cssa.cpp | 35 +++++++++++++++++---------
 1 file changed, 23 insertions(+), 12 deletions(-)

diff --git a/src/amd/compiler/aco_lower_to_cssa.cpp b/src/amd/compiler/aco_lower_to_cssa.cpp
index ff842cb7f0b..cbd9873c32b 100644
--- a/src/amd/compiler/aco_lower_to_cssa.cpp
+++ b/src/amd/compiler/aco_lower_to_cssa.cpp
@@ -445,6 +445,9 @@ emit_parallelcopies(cssa_ctx& ctx)
          continue;
 
       std::map<uint32_t, ltg_node> ltg;
+      bool has_vgpr_copy = false;
+      bool has_sgpr_copy = false;
+
       /* first, try to coalesce all parallelcopies */
       for (const copy& cp : ctx.parallelcopies[i]) {
          if (try_coalesce_copy(ctx, cp, i)) {
@@ -459,6 +462,10 @@ emit_parallelcopies(cssa_ctx& ctx)
             uint32_t write_idx = ctx.merge_node_table[cp.def.tempId()].index;
             assert(write_idx != -1u);
             ltg[write_idx] = {cp, read_idx};
+
+            bool is_vgpr = cp.def.regClass().type() == RegType::vgpr;
+            has_vgpr_copy |= is_vgpr;
+            has_sgpr_copy |= !is_vgpr;
          }
       }
 
@@ -475,19 +482,23 @@ emit_parallelcopies(cssa_ctx& ctx)
       Builder bld(ctx.program);
       Block& block = ctx.program->blocks[i];
 
-      /* emit VGPR copies */
-      auto IsLogicalEnd = [](const aco_ptr<Instruction>& inst) -> bool
-      { return inst->opcode == aco_opcode::p_logical_end; };
-      auto it = std::find_if(block.instructions.rbegin(), block.instructions.rend(), IsLogicalEnd);
-      bld.reset(&block.instructions, std::prev(it.base()));
-      emit_copies_block(bld, ltg, RegType::vgpr);
+      if (has_vgpr_copy) {
+         /* emit VGPR copies */
+         auto IsLogicalEnd = [](const aco_ptr<Instruction>& inst) -> bool
+         { return inst->opcode == aco_opcode::p_logical_end; };
+         auto it = std::find_if(block.instructions.rbegin(), block.instructions.rend(), IsLogicalEnd);
+         bld.reset(&block.instructions, std::prev(it.base()));
+         emit_copies_block(bld, ltg, RegType::vgpr);
+      }
 
-      /* emit SGPR copies */
-      aco_ptr<Instruction> branch = std::move(block.instructions.back());
-      block.instructions.pop_back();
-      bld.reset(&block.instructions);
-      emit_copies_block(bld, ltg, RegType::sgpr);
-      bld.insert(std::move(branch));
+      if (has_sgpr_copy) {
+         /* emit SGPR copies */
+         aco_ptr<Instruction> branch = std::move(block.instructions.back());
+         block.instructions.pop_back();
+         bld.reset(&block.instructions);
+         emit_copies_block(bld, ltg, RegType::sgpr);
+         bld.insert(std::move(branch));
+      }
    }
 
    /* finally, rename coalesced phi operands */