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-rw-r--r--src/dynarmic/docs/ReturnStackBufferOptimization.md176
1 files changed, 94 insertions, 82 deletions
diff --git a/src/dynarmic/docs/ReturnStackBufferOptimization.md b/src/dynarmic/docs/ReturnStackBufferOptimization.md
index 6ffe41bcc6..0e72c3bce8 100644
--- a/src/dynarmic/docs/ReturnStackBufferOptimization.md
+++ b/src/dynarmic/docs/ReturnStackBufferOptimization.md
@@ -23,15 +23,17 @@ One complication dynarmic has is that a compiled block is not uniquely identifia
the PC alone, but bits in the FPSCR and CPSR are also relevant. We resolve this by
computing a 64-bit `UniqueHash` that is guaranteed to uniquely identify a block.
- u64 LocationDescriptor::UniqueHash() const {
- // This value MUST BE UNIQUE.
- // This calculation has to match up with EmitX64::EmitTerminalPopRSBHint
- u64 pc_u64 = u64(arm_pc) << 32;
- u64 fpscr_u64 = u64(fpscr.Value());
- u64 t_u64 = cpsr.T() ? 1 : 0;
- u64 e_u64 = cpsr.E() ? 2 : 0;
- return pc_u64 | fpscr_u64 | t_u64 | e_u64;
- }
+```c++
+u64 LocationDescriptor::UniqueHash() const {
+ // This value MUST BE UNIQUE.
+ // This calculation has to match up with EmitX64::EmitTerminalPopRSBHint
+ u64 pc_u64 = u64(arm_pc) << 32;
+ u64 fpscr_u64 = u64(fpscr.Value());
+ u64 t_u64 = cpsr.T() ? 1 : 0;
+ u64 e_u64 = cpsr.E() ? 2 : 0;
+ return pc_u64 | fpscr_u64 | t_u64 | e_u64;
+}
+```
## Our implementation isn't actually a stack
@@ -49,97 +51,107 @@ host addresses for the corresponding the compiled blocks.
size of the real RSB in hardware (which has 3 entries). Larger RSBs than 8
showed degraded performance.
- struct JitState {
- // ...
+```c++
+struct JitState {
+ // ...
- static constexpr size_t RSBSize = 8; // MUST be a power of 2.
- u32 rsb_ptr = 0;
- std::array<u64, RSBSize> rsb_location_descriptors;
- std::array<u64, RSBSize> rsb_codeptrs;
- void ResetRSB();
+ static constexpr size_t RSBSize = 8; // MUST be a power of 2.
+ u32 rsb_ptr = 0;
+ std::array<u64, RSBSize> rsb_location_descriptors;
+ std::array<u64, RSBSize> rsb_codeptrs;
+ void ResetRSB();
- // ...
- };
+ // ...
+};
+```
### RSB Push
We insert our prediction at the insertion point iff the RSB doesn't already
contain a prediction with the same `UniqueHash`.
- void EmitX64::EmitPushRSB(IR::Block&, IR::Inst* inst) {
- using namespace Xbyak::util;
-
- ASSERT(inst->GetArg(0).IsImmediate());
- u64 imm64 = inst->GetArg(0).GetU64();
-
- Xbyak::Reg64 code_ptr_reg = reg_alloc.ScratchGpr({HostLoc::RCX});
- Xbyak::Reg64 loc_desc_reg = reg_alloc.ScratchGpr();
- Xbyak::Reg32 index_reg = reg_alloc.ScratchGpr().cvt32();
- u64 code_ptr = unique_hash_to_code_ptr.find(imm64) != unique_hash_to_code_ptr.end()
- ? u64(unique_hash_to_code_ptr[imm64])
- : u64(code->GetReturnFromRunCodeAddress());
-
- code->mov(index_reg, dword[code.ABI_JIT_PTR + offsetof(JitState, rsb_ptr)]);
- code->add(index_reg, 1);
- code->and_(index_reg, u32(JitState::RSBSize - 1));
-
- code->mov(loc_desc_reg, u64(imm64));
- CodePtr patch_location = code->getCurr<CodePtr>();
- patch_unique_hash_locations[imm64].emplace_back(patch_location);
- code->mov(code_ptr_reg, u64(code_ptr)); // This line has to match up with EmitX64::Patch.
- code->EnsurePatchLocationSize(patch_location, 10);
-
- Xbyak::Label label;
- for (size_t i = 0; i < JitState::RSBSize; ++i) {
- code->cmp(loc_desc_reg, qword[code.ABI_JIT_PTR + offsetof(JitState, rsb_location_descriptors) + i * sizeof(u64)]);
- code->je(label, code->T_SHORT);
- }
-
- code->mov(dword[code.ABI_JIT_PTR + offsetof(JitState, rsb_ptr)], index_reg);
- code->mov(qword[code.ABI_JIT_PTR + index_reg.cvt64() * 8 + offsetof(JitState, rsb_location_descriptors)], loc_desc_reg);
- code->mov(qword[code.ABI_JIT_PTR + index_reg.cvt64() * 8 + offsetof(JitState, rsb_codeptrs)], code_ptr_reg);
- code->L(label);
+```c++
+void EmitX64::EmitPushRSB(IR::Block&, IR::Inst* inst) {
+ using namespace Xbyak::util;
+
+ ASSERT(inst->GetArg(0).IsImmediate());
+ u64 imm64 = inst->GetArg(0).GetU64();
+
+ Xbyak::Reg64 code_ptr_reg = reg_alloc.ScratchGpr({HostLoc::RCX});
+ Xbyak::Reg64 loc_desc_reg = reg_alloc.ScratchGpr();
+ Xbyak::Reg32 index_reg = reg_alloc.ScratchGpr().cvt32();
+ u64 code_ptr = unique_hash_to_code_ptr.find(imm64) != unique_hash_to_code_ptr.end()
+ ? u64(unique_hash_to_code_ptr[imm64])
+ : u64(code->GetReturnFromRunCodeAddress());
+
+ code->mov(index_reg, dword[code.ABI_JIT_PTR + offsetof(JitState, rsb_ptr)]);
+ code->add(index_reg, 1);
+ code->and_(index_reg, u32(JitState::RSBSize - 1));
+
+ code->mov(loc_desc_reg, u64(imm64));
+ CodePtr patch_location = code->getCurr<CodePtr>();
+ patch_unique_hash_locations[imm64].emplace_back(patch_location);
+ code->mov(code_ptr_reg, u64(code_ptr)); // This line has to match up with EmitX64::Patch.
+ code->EnsurePatchLocationSize(patch_location, 10);
+
+ Xbyak::Label label;
+ for (size_t i = 0; i < JitState::RSBSize; ++i) {
+ code->cmp(loc_desc_reg, qword[code.ABI_JIT_PTR + offsetof(JitState, rsb_location_descriptors) + i * sizeof(u64)]);
+ code->je(label, code->T_SHORT);
}
+ code->mov(dword[code.ABI_JIT_PTR + offsetof(JitState, rsb_ptr)], index_reg);
+ code->mov(qword[code.ABI_JIT_PTR + index_reg.cvt64() * 8 + offsetof(JitState, rsb_location_descriptors)], loc_desc_reg);
+ code->mov(qword[code.ABI_JIT_PTR + index_reg.cvt64() * 8 + offsetof(JitState, rsb_codeptrs)], code_ptr_reg);
+ code->L(label);
+}
+```
+
In pseudocode:
- for (i := 0 .. RSBSize-1)
- if (rsb_location_descriptors[i] == imm64)
- goto label;
- rsb_ptr++;
- rsb_ptr %= RSBSize;
- rsb_location_desciptors[rsb_ptr] = imm64; //< The UniqueHash
- rsb_codeptr[rsb_ptr] = /* codeptr corresponding to the UniqueHash */;
- label:
+```c++
+ for (i := 0 .. RSBSize-1)
+ if (rsb_location_descriptors[i] == imm64)
+ goto label;
+ rsb_ptr++;
+ rsb_ptr %= RSBSize;
+ rsb_location_desciptors[rsb_ptr] = imm64; //< The UniqueHash
+ rsb_codeptr[rsb_ptr] = /* codeptr corresponding to the UniqueHash */;
+label:
+```
## RSB Pop
To check if a predicition is in the RSB, we linearly scan the RSB.
- void EmitX64::EmitTerminalPopRSBHint(IR::Term::PopRSBHint, IR::LocationDescriptor initial_location) {
- using namespace Xbyak::util;
-
- // This calculation has to match up with IREmitter::PushRSB
- code->mov(ecx, MJitStateReg(Arm::Reg::PC));
- code->shl(rcx, 32);
- code->mov(ebx, dword[code.ABI_JIT_PTR + offsetof(JitState, FPSCR_mode)]);
- code->or_(ebx, dword[code.ABI_JIT_PTR + offsetof(JitState, CPSR_et)]);
- code->or_(rbx, rcx);
-
- code->mov(rax, u64(code->GetReturnFromRunCodeAddress()));
- for (size_t i = 0; i < JitState::RSBSize; ++i) {
- code->cmp(rbx, qword[code.ABI_JIT_PTR + offsetof(JitState, rsb_location_descriptors) + i * sizeof(u64)]);
- code->cmove(rax, qword[code.ABI_JIT_PTR + offsetof(JitState, rsb_codeptrs) + i * sizeof(u64)]);
- }
-
- code->jmp(rax);
+```c++
+void EmitX64::EmitTerminalPopRSBHint(IR::Term::PopRSBHint, IR::LocationDescriptor initial_location) {
+ using namespace Xbyak::util;
+
+ // This calculation has to match up with IREmitter::PushRSB
+ code->mov(ecx, MJitStateReg(Arm::Reg::PC));
+ code->shl(rcx, 32);
+ code->mov(ebx, dword[code.ABI_JIT_PTR + offsetof(JitState, FPSCR_mode)]);
+ code->or_(ebx, dword[code.ABI_JIT_PTR + offsetof(JitState, CPSR_et)]);
+ code->or_(rbx, rcx);
+
+ code->mov(rax, u64(code->GetReturnFromRunCodeAddress()));
+ for (size_t i = 0; i < JitState::RSBSize; ++i) {
+ code->cmp(rbx, qword[code.ABI_JIT_PTR + offsetof(JitState, rsb_location_descriptors) + i * sizeof(u64)]);
+ code->cmove(rax, qword[code.ABI_JIT_PTR + offsetof(JitState, rsb_codeptrs) + i * sizeof(u64)]);
}
+ code->jmp(rax);
+}
+```
+
In pseudocode:
- rbx := ComputeUniqueHash()
- rax := ReturnToDispatch
- for (i := 0 .. RSBSize-1)
- if (rbx == rsb_location_descriptors[i])
- rax = rsb_codeptrs[i]
- goto rax \ No newline at end of file
+```c++
+rbx := ComputeUniqueHash()
+rax := ReturnToDispatch
+for (i := 0 .. RSBSize-1)
+ if (rbx == rsb_location_descriptors[i])
+ rax = rsb_codeptrs[i]
+goto rax
+```
generated by cgit v1.2.3 (git 2.39.1) at 2026-11-04 02:45:26 +0000