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[MCA][X86] Pretend To Have a Stack Engine #153348

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20 changes: 20 additions & 0 deletions llvm/lib/Target/X86/MCA/X86CustomBehaviour.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -36,11 +36,31 @@ void X86InstrPostProcess::setMemBarriers(std::unique_ptr<Instruction> &Inst,
}
}

void X86InstrPostProcess::useStackEngine(std::unique_ptr<Instruction> &Inst,
const MCInst &MCI) {
if (X86::isPOP(MCI.getOpcode())) {
assert(Inst->getUses().size() == 1 &&
"Expected pop instruction to only use stack pointer register");
Inst->getUses().clear();
}
Comment on lines +41 to +45
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@adibiagio adibiagio Aug 13, 2025 •
edited
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A use of RSP is "free" only if the definition of RSP comes from another PUSH/POP/RET. That is because the stack engine only tracks RSP updates from those known stack opcodes. The stack engine computes a delta between the new RSP value and the old RSP value. That delta is then used as an implicit addend operand to other RSP reads performed by other stack operations.

The first time a stack operation is encountered, RSP should be treated like a normal read dependency, and we would pay the full cost of waiting on RSP. Any other following stack operations will be able to eliminate their dependency from RSP.

A write to RSP which isn't from a stack operation would force a synchronization. So, the next stack operation won't be able to benefit from that optimization. We would basically need to go back to square one.

In your case:

  • if the last definition of RSP is unknown (we just started simulating), or it is not coming from another PUSH/POP/RET, then we should pay the full dependency cost.
  • if there is a definition of RSP which isn't coming from a stack operation, then the next PUSH/POP/RET will pay the full cost of waiting on RSP.

Bottom line: your change is mostly OK. However, you need to add some sort of post-processing state/context;
as you post-process instructions, you should record where the last definition of RSP came from. Your logic would only eliminate the dependency if the definition was coming from a known stack operation.

I hope it makes sense.
-Andrea

if (X86::isPUSH(MCI.getOpcode())) {
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@adibiagio adibiagio Aug 13, 2025 •
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NOTE: there is also PUSHF and POPF (for the purpose of this analysis, you can treat them like normal PUSH/POP).
Ignore RET for now.. that's problematic for other reasons, and we don't simulate it properly.

Edit: on a second thought, we don't know whether PUSHF/POPF are also optimized, or whether they force a synchronization. That needs to be tested on real hw. For now, I think you can ignore them.

auto *StackRegisterUse =
llvm::find_if(Inst->getUses(), [](const ReadState &State) {
return State.getRegisterID() == X86::RSP;
});
assert(
StackRegisterUse != Inst->getUses().end() &&
"Expected push instruction to implicitly use stack pointer register.");
Inst->getUses().erase(StackRegisterUse);
}
}

void X86InstrPostProcess::postProcessInstruction(
std::unique_ptr<Instruction> &Inst, const MCInst &MCI) {
// Currently, we only modify certain instructions' IsALoadBarrier and
// IsAStoreBarrier flags.
setMemBarriers(Inst, MCI);
useStackEngine(Inst, MCI);
}

} // namespace mca
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5 changes: 5 additions & 0 deletions llvm/lib/Target/X86/MCA/X86CustomBehaviour.h
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Original file line number Diff line number Diff line change
Expand Up @@ -28,6 +28,11 @@ class X86InstrPostProcess : public InstrPostProcess {
/// as load and store barriers.
void setMemBarriers(std::unique_ptr<Instruction> &Inst, const MCInst &MCI);

/// Called within X86InstrPostPorcess to remove some rsp read operands
/// on stack instructions to better simulate the stack engine. We currently
/// do not model features of the stack engine like sync uops.
void useStackEngine(std::unique_ptr<Instruction> &Inst, const MCInst &MCI);

public:
X86InstrPostProcess(const MCSubtargetInfo &STI, const MCInstrInfo &MCII)
: InstrPostProcess(STI, MCII) {}
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81 changes: 81 additions & 0 deletions llvm/test/tools/llvm-mca/X86/stack-engine-pop.s
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@@ -0,0 +1,81 @@
# NOTE: Assertions have been autogenerated by utils/update_mca_test_checks.py
# RUN: llvm-mca -mtriple=x86_64-unknown-unknown -mcpu=skylake -timeline -iterations=1 < %s | FileCheck %s

popq %rax
popq %rcx
popq %rdx
popq %rbx
popq %r12

# CHECK: Iterations: 1
# CHECK-NEXT: Instructions: 5
# CHECK-NEXT: Total Cycles: 11
# CHECK-NEXT: Total uOps: 10

# CHECK: Dispatch Width: 6
# CHECK-NEXT: uOps Per Cycle: 0.91
# CHECK-NEXT: IPC: 0.45
# CHECK-NEXT: Block RThroughput: 2.5

# CHECK: Instruction Info:
# CHECK-NEXT: [1]: #uOps
# CHECK-NEXT: [2]: Latency
# CHECK-NEXT: [3]: RThroughput
# CHECK-NEXT: [4]: MayLoad
# CHECK-NEXT: [5]: MayStore
# CHECK-NEXT: [6]: HasSideEffects (U)

# CHECK: [1] [2] [3] [4] [5] [6] Instructions:
# CHECK-NEXT: 2 6 0.50 * popq %rax
# CHECK-NEXT: 2 6 0.50 * popq %rcx
# CHECK-NEXT: 2 6 0.50 * popq %rdx
# CHECK-NEXT: 2 6 0.50 * popq %rbx
# CHECK-NEXT: 2 6 0.50 * popq %r12

# CHECK: Resources:
# CHECK-NEXT: [0] - SKLDivider
# CHECK-NEXT: [1] - SKLFPDivider
# CHECK-NEXT: [2] - SKLPort0
# CHECK-NEXT: [3] - SKLPort1
# CHECK-NEXT: [4] - SKLPort2
# CHECK-NEXT: [5] - SKLPort3
# CHECK-NEXT: [6] - SKLPort4
# CHECK-NEXT: [7] - SKLPort5
# CHECK-NEXT: [8] - SKLPort6
# CHECK-NEXT: [9] - SKLPort7

# CHECK: Resource pressure per iteration:
# CHECK-NEXT: [0] [1] [2] [3] [4] [5] [6] [7] [8] [9]
# CHECK-NEXT: - - 1.00 1.00 2.00 3.00 - 1.00 2.00 -

# CHECK: Resource pressure by instruction:
# CHECK-NEXT: [0] [1] [2] [3] [4] [5] [6] [7] [8] [9] Instructions:
# CHECK-NEXT: - - - - - 1.00 - - 1.00 - popq %rax
# CHECK-NEXT: - - - - 1.00 - - 1.00 - - popq %rcx
# CHECK-NEXT: - - - 1.00 - 1.00 - - - - popq %rdx
# CHECK-NEXT: - - 1.00 - 1.00 - - - - - popq %rbx
# CHECK-NEXT: - - - - - 1.00 - - 1.00 - popq %r12

# CHECK: Timeline view:
# CHECK-NEXT: 0
# CHECK-NEXT: Index 0123456789

# CHECK: [0,0] DeeeeeeER . popq %rax
# CHECK-NEXT: [0,1] DeeeeeeER . popq %rcx
# CHECK-NEXT: [0,2] D=eeeeeeER. popq %rdx
# CHECK-NEXT: [0,3] .DeeeeeeER. popq %rbx
# CHECK-NEXT: [0,4] .D=eeeeeeER popq %r12

# CHECK: Average Wait times (based on the timeline view):
# CHECK-NEXT: [0]: Executions
# CHECK-NEXT: [1]: Average time spent waiting in a scheduler's queue
# CHECK-NEXT: [2]: Average time spent waiting in a scheduler's queue while ready
# CHECK-NEXT: [3]: Average time elapsed from WB until retire stage

# CHECK: [0] [1] [2] [3]
# CHECK-NEXT: 0. 1 1.0 1.0 0.0 popq %rax
# CHECK-NEXT: 1. 1 1.0 1.0 0.0 popq %rcx
# CHECK-NEXT: 2. 1 2.0 2.0 0.0 popq %rdx
# CHECK-NEXT: 3. 1 1.0 1.0 0.0 popq %rbx
# CHECK-NEXT: 4. 1 2.0 2.0 0.0 popq %r12
# CHECK-NEXT: 1 1.4 1.4 0.0 <total>
80 changes: 80 additions & 0 deletions llvm/test/tools/llvm-mca/X86/stack-engine-push.s
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@@ -0,0 +1,80 @@
# NOTE: Assertions have been autogenerated by utils/update_mca_test_checks.py
# RUN: llvm-mca -mtriple=x86_64-unknown-unknown -mcpu=skylake -timeline -iterations=1 < %s | FileCheck %s

pushq %rax
pushq %rcx
pushq %rdx
pushq %rbx
pushq %r12

# CHECK: Iterations: 1
# CHECK-NEXT: Instructions: 5
# CHECK-NEXT: Total Cycles: 9
# CHECK-NEXT: Total uOps: 15

# CHECK: Dispatch Width: 6
# CHECK-NEXT: uOps Per Cycle: 1.67
# CHECK-NEXT: IPC: 0.56
# CHECK-NEXT: Block RThroughput: 5.0

# CHECK: Instruction Info:
# CHECK-NEXT: [1]: #uOps
# CHECK-NEXT: [2]: Latency
# CHECK-NEXT: [3]: RThroughput
# CHECK-NEXT: [4]: MayLoad
# CHECK-NEXT: [5]: MayStore
# CHECK-NEXT: [6]: HasSideEffects (U)

# CHECK: [1] [2] [3] [4] [5] [6] Instructions:
# CHECK-NEXT: 3 2 1.00 * pushq %rax
# CHECK-NEXT: 3 2 1.00 * pushq %rcx
# CHECK-NEXT: 3 2 1.00 * pushq %rdx
# CHECK-NEXT: 3 2 1.00 * pushq %rbx
# CHECK-NEXT: 3 2 1.00 * pushq %r12

# CHECK: Resources:
# CHECK-NEXT: [0] - SKLDivider
# CHECK-NEXT: [1] - SKLFPDivider
# CHECK-NEXT: [2] - SKLPort0
# CHECK-NEXT: [3] - SKLPort1
# CHECK-NEXT: [4] - SKLPort2
# CHECK-NEXT: [5] - SKLPort3
# CHECK-NEXT: [6] - SKLPort4
# CHECK-NEXT: [7] - SKLPort5
# CHECK-NEXT: [8] - SKLPort6
# CHECK-NEXT: [9] - SKLPort7

# CHECK: Resource pressure per iteration:
# CHECK-NEXT: [0] [1] [2] [3] [4] [5] [6] [7] [8] [9]
# CHECK-NEXT: - - 1.00 1.00 1.00 2.00 5.00 1.00 2.00 2.00

# CHECK: Resource pressure by instruction:
# CHECK-NEXT: [0] [1] [2] [3] [4] [5] [6] [7] [8] [9] Instructions:
# CHECK-NEXT: - - - - - - 1.00 - 1.00 1.00 pushq %rax
# CHECK-NEXT: - - - - - 1.00 1.00 1.00 - - pushq %rcx
# CHECK-NEXT: - - - 1.00 1.00 - 1.00 - - - pushq %rdx
# CHECK-NEXT: - - 1.00 - - - 1.00 - - 1.00 pushq %rbx
# CHECK-NEXT: - - - - - 1.00 1.00 - 1.00 - pushq %r12

# CHECK: Timeline view:
# CHECK-NEXT: Index 012345678

# CHECK: [0,0] DeeER. . pushq %rax
# CHECK-NEXT: [0,1] D=eeER . pushq %rcx
# CHECK-NEXT: [0,2] .D=eeER . pushq %rdx
# CHECK-NEXT: [0,3] .D==eeER. pushq %rbx
# CHECK-NEXT: [0,4] . D==eeER pushq %r12

# CHECK: Average Wait times (based on the timeline view):
# CHECK-NEXT: [0]: Executions
# CHECK-NEXT: [1]: Average time spent waiting in a scheduler's queue
# CHECK-NEXT: [2]: Average time spent waiting in a scheduler's queue while ready
# CHECK-NEXT: [3]: Average time elapsed from WB until retire stage

# CHECK: [0] [1] [2] [3]
# CHECK-NEXT: 0. 1 1.0 1.0 0.0 pushq %rax
# CHECK-NEXT: 1. 1 2.0 1.0 0.0 pushq %rcx
# CHECK-NEXT: 2. 1 2.0 1.0 0.0 pushq %rdx
# CHECK-NEXT: 3. 1 3.0 1.0 0.0 pushq %rbx
# CHECK-NEXT: 4. 1 3.0 1.0 0.0 pushq %r12
# CHECK-NEXT: 1 2.2 1.0 0.0 <total>