aco/gfx10: Mitigate LdsBranchVmemWARHazard.

There is a hazard caused by there is a branch between a
VMEM/GLOBAL/SCRATCH instruction and a DS instruction.
This commit adds a workaround that avoids the problem.

Signed-off-by: Timur Kristóf <timur.kristof@gmail.com>
Reviewed-by: Daniel Schürmann <daniel@schuermann.dev>

diff --git a/src/amd/compiler/README b/src/amd/compiler/README
index 620b4bc..822ecd7 100644 (file)
--- a/src/amd/compiler/README
+++ b/src/amd/compiler/README
@@ -200,3 +200,12 @@ Mitigated by:
 A VALU instruction that writes an SGPR (or has a valid SDST operand), or `s_waitcnt_depctr 0xfffe`.
 Note: `s_waitcnt_depctr` is an internal instruction, so there is no further information
 about what it does or what its operand means.
+
+### LdsBranchVmemWARHazard
+
+Triggered by:
+VMEM/GLOBAL/SCRATCH instruction, then a branch, then a DS instruction,
+or vice versa: DS instruction, then a branch, then a VMEM/GLOBAL/SCRATCH instruction.
+
+Mitigated by:
+Only `s_waitcnt_vscnt null, 0`. Needed even if the first instruction is a load.

diff --git a/src/amd/compiler/aco_insert_NOPs.cpp b/src/amd/compiler/aco_insert_NOPs.cpp
index 6924ea1..2442f60 100644 (file)
--- a/src/amd/compiler/aco_insert_NOPs.cpp
+++ b/src/amd/compiler/aco_insert_NOPs.cpp
@@ -43,6 +43,10 @@ struct NOP_ctx {
    int last_VMEM_since_scalar_write = -1;
    bool has_VOPC = false;
    bool has_nonVALU_exec_read = false;
+   bool has_VMEM = false;
+   bool has_branch_after_VMEM = false;
+   bool has_DS = false;
+   bool has_branch_after_DS = false;
    std::bitset<128> sgprs_read_by_SMEM;
 
    NOP_ctx(Program* program) : chip_class(program->chip_class) {
@@ -107,6 +111,27 @@ bool instr_writes_sgpr(const aco_ptr<Instruction>& instr)
    });
 }
 
+inline bool instr_is_branch(const aco_ptr<Instruction>& instr)
+{
+   return instr->opcode == aco_opcode::s_branch ||
+          instr->opcode == aco_opcode::s_cbranch_scc0 ||
+          instr->opcode == aco_opcode::s_cbranch_scc1 ||
+          instr->opcode == aco_opcode::s_cbranch_vccz ||
+          instr->opcode == aco_opcode::s_cbranch_vccnz ||
+          instr->opcode == aco_opcode::s_cbranch_execz ||
+          instr->opcode == aco_opcode::s_cbranch_execnz ||
+          instr->opcode == aco_opcode::s_cbranch_cdbgsys ||
+          instr->opcode == aco_opcode::s_cbranch_cdbguser ||
+          instr->opcode == aco_opcode::s_cbranch_cdbgsys_or_user ||
+          instr->opcode == aco_opcode::s_cbranch_cdbgsys_and_user ||
+          instr->opcode == aco_opcode::s_subvector_loop_begin ||
+          instr->opcode == aco_opcode::s_subvector_loop_end ||
+          instr->opcode == aco_opcode::s_setpc_b64 ||
+          instr->opcode == aco_opcode::s_swappc_b64 ||
+          instr->opcode == aco_opcode::s_getpc_b64 ||
+          instr->opcode == aco_opcode::s_call_b64;
+}
+
 bool regs_intersect(PhysReg a_reg, unsigned a_size, PhysReg b_reg, unsigned b_size)
 {
    return a_reg > b_reg ?
@@ -413,6 +438,38 @@ std::pair<int, int> handle_instruction_gfx10(NOP_ctx& ctx, aco_ptr<Instruction>&
       }
    }
 
+   /* LdsBranchVmemWARHazard
+    * Handle VMEM/GLOBAL/SCRATCH->branch->DS and DS->branch->VMEM/GLOBAL/SCRATCH patterns.
+    */
+   if (instr->isVMEM() || instr->format == Format::GLOBAL || instr->format == Format::SCRATCH) {
+      ctx.has_VMEM = true;
+      ctx.has_branch_after_VMEM = false;
+      /* Mitigation for DS is needed only if there was already a branch after */
+      ctx.has_DS = ctx.has_branch_after_DS;
+   } else if (instr->format == Format::DS) {
+      ctx.has_DS = true;
+      ctx.has_branch_after_DS = false;
+      /* Mitigation for VMEM is needed only if there was already a branch after */
+      ctx.has_VMEM = ctx.has_branch_after_VMEM;
+   } else if (instr_is_branch(instr)) {
+      ctx.has_branch_after_VMEM = ctx.has_VMEM;
+      ctx.has_branch_after_DS = ctx.has_DS;
+   } else if (instr->opcode == aco_opcode::s_waitcnt_vscnt) {
+      /* Only s_waitcnt_vscnt can mitigate the hazard */
+      const SOPK_instruction *sopk = static_cast<const SOPK_instruction *>(instr.get());
+      if (sopk->definitions[0].physReg() == sgpr_null && sopk->imm == 0)
+         ctx.has_VMEM = ctx.has_branch_after_VMEM = ctx.has_DS = ctx.has_branch_after_DS = false;
+   }
+   if ((ctx.has_VMEM && ctx.has_branch_after_DS) || (ctx.has_DS && ctx.has_branch_after_VMEM)) {
+      ctx.has_VMEM = ctx.has_branch_after_VMEM = ctx.has_DS = ctx.has_branch_after_DS = false;
+
+      /* Insert s_waitcnt_vscnt to mitigate the problem */
+      aco_ptr<SOPK_instruction> wait{create_instruction<SOPK_instruction>(aco_opcode::s_waitcnt_vscnt, Format::SOPK, 0, 1)};
+      wait->definitions[0] = Definition(sgpr_null, s1);
+      wait->imm = 0;
+      new_instructions.emplace_back(std::move(wait));
+   }
+
    return std::make_pair(sNOPs, vNOPs);
 }