liblzma: memcmplen.h: Add a comment why subtraction is used.
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@ -67,6 +67,19 @@ lzma_memcmplen(const uint8_t *buf1, const uint8_t *buf2,
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// This is only for x86-64 and ARM64 for now. This might be fine on
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// other 64-bit processors too. On big endian one should use xor
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// instead of subtraction and switch to __builtin_clzll().
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//
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// Reasons to use subtraction instead of xor:
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//
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// - On some x86-64 processors (Intel Sandy Bridge to Tiger Lake),
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// sub+jz and sub+jnz can be fused but xor+jz or xor+jnz cannot.
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// Thus using subtraction has potential to be a tiny amount faster
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// since the code checks if the quotient is non-zero.
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//
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// - Some processors (Intel Pentium 4) used to have more ALU
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// resources for add/sub instructions than and/or/xor.
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//
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// The processor info is based on Agner Fog's microarchitecture.pdf
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// version 2023-05-26. https://www.agner.org/optimize/
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#define LZMA_MEMCMPLEN_EXTRA 8
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while (len < limit) {
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const uint64_t x = read64ne(buf1 + len) - read64ne(buf2 + len);
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