JitArm64_Integer: cmp/cmpl optimizations #13324
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You can encode a 12-bit immediate in a SUB instruction on ARM64. Constants in this range do not need to be sign extended, so we can exploit this to avoid materializing the immediate. This approach saves an instruction if it does not need to be materialized in a register afterwards. Otherwise, we just materialize it later and the total number of instructions stays the same. Before: 0x52800416 mov w22, #0x20 ; =32 0x93407f78 sxtw x24, w27 0xcb36c318 sub x24, x24, w22, sxtw After: 0x93407f78 sxtw x24, w27 0xd1008318 sub x24, x24, #0x20
You can encode a shifted 12-bit immediate in a SUB instruction on ARM64. Constants in this range do not need to be sign extended, so we can exploit this to avoid materializing the immediate. This approach saves an instruction if it does not need to be materialized in a register afterwards. Otherwise, we just materialize it later and the total number of instructions stays the same. Before: 0x52a00099 mov w25, #0x40000 ; =262144 0x93407f7a sxtw x26, w27 0xcb39c35a sub x26, x26, w25, sxtw After: 0x93407f7a sxtw x26, w27 0xd141035a sub x26, x26, #0x40, lsl dolphin-emu#12 ; =0x40000
You can encode a 12-bit immediate in an ADD instruction on ARM64. If the negated constant fits in this range, we can exploit this to avoid materializing the immediate. This approach saves an instruction if it does not need to be materialized in a register afterwards. Otherwise, we just materialize it later and the total number of instructions stays the same. Before: 0x12800019 mov w25, #-0x1 ; =-1 0x93407f5b sxtw x27, w26 0xcb39c37b sub x27, x27, w25, sxtw After: 0x93407f5b sxtw x27, w26 0x9100077b add x27, x27, #0x1
You can encode a shifted 12-bit immediate in an ADD instruction on ARM64. If the negated constant fits in this range, we can exploit this to avoid materializing the immediate. This approach saves an instruction if it does not need to be materialized in a register afterwards. Otherwise, we just materialize it later and the total number of instructions stays the same. Before: 0x52bff01a mov w26, #-0x800000 ; =-8388608 0x93407f1b sxtw x27, w24 0xcb3ac37b sub x27, x27, w26, sxtw After: 0x93407f1b sxtw x27, w24 0x9160037b add x27, x27, #0x800, lsl dolphin-emu#12 ; =0x800000
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While we cannot always avoid materializing immediates, we can still inspect the most significant bit and potentially skip sign extension. This can sometimes save an instruction. Before: 0x5280003a mov w26, #0x1 ; =1 0x93407f5b sxtw x27, w26 0xcb38c37b sub x27, x27, w24, sxtw After: 0x5280003a mov w26, #0x1 ; =1 0xcb38c35b sub x27, x26, w24, sxtw Before: 0x52a20018 mov w24, #0x10000000 ; =268435456 0x93407f79 sxtw x25, w27 0xcb38c339 sub x25, x25, w24, sxtw After: 0x52a20018 mov w24, #0x10000000 ; =268435456 0x93407f79 sxtw x25, w27 0xcb180339 sub x25, x25, x24
You can encode a 12-bit immediate in a SUB instruction on ARM64. We can exploit this to avoid materializing the immediate. This approach saves an instruction if it does not need to be materialized in a register afterwards. Otherwise, we just materialize it later and the total number of instructions stays the same. Before: 0x5280003a mov w26, #0x1 ; =1 0xcb1a033b sub x27, x25, x26 After: 0xd100073b sub x27, x25, #0x1
You can encode a shifted 12-bit immediate in a SUB instruction on ARM64. We exploit this to avoid materializing the immediate. This approach saves an instruction if it does not need to be materialized in a register afterwards. Otherwise, we just materialize it later and the total number of instructions stays the same. Before: 0x52a00218 mov w24, #0x100000 ; =1048576 0xcb180379 sub x25, x27, x24 After: 0xd1440379 sub x25, x27, #0x100, lsl dolphin-emu#12 ; =0x100000
Forgot this when I added it in dolphin-emu#13120.
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The main optimizations here take advantage of ARM64's
ADD/SUBinstructions which can encode a (shifted) 12-bit immediate value, allowing us to skip materializing the immediate in a register. This either means we save an instruction, we end up with an equal amount of instructions if it needs to be materialized for a subsequence use. I didn't implement the (shifted) 12-bit additions forcmpl, as I was unable to find games where this occurred.The second optimization consists of skipping the sign extension step in
cmpif possible.Finally, I fixed a const correctness issue I accidentally introduced in #13120.
cmp - Subtract 12-bit constant
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cmp - Subtract shifted 12-bit constant
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cmp - Add 12-bit constant
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cmp - Add shifted 12-bit constant
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cmp - Skip sign extension for a
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cmp - Skip sign extension for b
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cmpl - Subtract 12-bit constant
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cmpl - Subtract shifted 12-bit constant
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