Jits: Discard registers which we know will be overwritten #9351
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merryhime
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Dec 26, 2020
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x64 changes lgtm |
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degasus
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Jan 27, 2021
degasus
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Jan 27, 2021
degasus
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Jan 27, 2021
| @@ -551,6 +551,10 @@ void PPCAnalyzer::SetInstructionStats(CodeBlock* block, CodeOp* code, const Gekk | |||
| code->outputFPRF = (opinfo->flags & FL_SET_FPRF) != 0; | |||
| code->canEndBlock = (opinfo->flags & FL_ENDBLOCK) != 0; | |||
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| // TODO: Is it possible to determine that some FPU instructions never cause exceptions? | |||
| code->canCauseException = | |||
| (opinfo->flags & (FL_LOADSTORE | FL_USE_FPU | FL_PROGRAMEXCEPTION)) != 0; | |||
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The first FPU instruction will throw an exception, so all others are fine.
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Can't FPU instructions throw exceptions in other situations as well (that may not have been implemented in the JITs yet), like division by zero? (See the discussion about True Crime on IRC earlier this month)
degasus
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Jan 27, 2021
| if (op.canEndBlock || op.canCauseException) | ||
| { | ||
| gprDiscardable = BitSet32{}; | ||
| fprDiscardable = BitSet32{}; |
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I wonder, is there a reason why wantsCR, wantsFPRF, ..., is only set for op.canEndBlock?
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I'm not sure exactly how the logic works, and I'm also not sure if it has to do with this PR.
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degasus
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We've run out of space :(
This commit adds a new "discarded" state for registers. Discarding a register is like flushing it, but without actually writing its value back to memory. We can discard a register only when it is guaranteed that no instruction will read from the register before it is next written to. Discarding reduces the register pressure a little, and can also let us skip a few flushes on interpreter fallbacks.
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This pull request adds a new "discarded" state for registers. Discarding a register is like flushing it, but without actually writing its value back to memory. We can discard a register only when it is guaranteed that no instruction will read from the register before it is next written to.
Discarding reduces the register pressure a little, and can also let us skip a few flushes on interpreter fallbacks.