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authorLasse Collin <lasse.collin@tukaani.org>2022-11-14 21:34:57 +0200
committerLasse Collin <lasse.collin@tukaani.org>2022-11-14 23:05:46 +0200
commitf644473a211394447824ea00518d0a214ff3f7f2 (patch)
tree8fc19fa7f3811b090f3f73398ce8bd0708d10a53 /INSTALL
parentTranslations: Update the Swedish translation one more time. (diff)
downloadxz-f644473a211394447824ea00518d0a214ff3f7f2.tar.xz
liblzma: Add fast CRC64 for 32/64-bit x86 using SSSE3 + SSE4.1 + CLMUL.
It also works on E2K as it supports these intrinsics. On x86-64 runtime detection is used so the code keeps working on older processors too. A CLMUL-only build can be done by using -msse4.1 -mpclmul in CFLAGS and this will reduce the library size since the generic implementation and its 8 KiB lookup table will be omitted. On 32-bit x86 this isn't used by default for now because by default on 32-bit x86 the separate assembly file crc64_x86.S is used. If --disable-assembler is used then this new CLMUL code is used the same way as on 64-bit x86. However, a CLMUL-only build (-msse4.1 -mpclmul) won't omit the 8 KiB lookup table on 32-bit x86 due to a currently-missing check for disabled assembler usage. The configure.ac check should be such that the code won't be built if something in the toolchain doesn't support it but --disable-clmul-crc option can be used to unconditionally disable this feature. CLMUL speeds up decompression of files that have compressed very well (assuming CRC64 is used as a check type). It is know that the CLMUL code is significantly slower than the generic code for tiny inputs (especially 1-8 bytes but up to 16 bytes). If that is a real-world problem then there is already a commented-out variant that uses the generic version for small inputs. Thanks to Ilya Kurdyukov for the original patch which was derived from a white paper from Intel [1] (published in 2009) and public domain code from [2] (released in 2016). [1] https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/fast-crc-computation-generic-polynomials-pclmulqdq-paper.pdf [2] https://github.com/rawrunprotected/crc
Diffstat (limited to 'INSTALL')
-rw-r--r--INSTALL12
1 files changed, 12 insertions, 0 deletions
diff --git a/INSTALL b/INSTALL
index 2c94ecea..bf1e9c31 100644
--- a/INSTALL
+++ b/INSTALL
@@ -370,6 +370,18 @@ XZ Utils Installation
pre-i686 systems, you may want to disable the assembler
code.
+ --disable-clmul-crc
+ Disable the use carryless multiplication for CRC
+ calculation even if compiler support for it is detected.
+ The code uses runtime detection of SSSE3, SSE4.1, and
+ CLMUL instructions on x86. On 32-bit x86 this currently
+ is used only if --disable-assembler is used (this might
+ be fixed in the future). The code works on E2K too.
+
+ If using compiler options that unconditionally allow the
+ required extensions (-msse4.1 -mpclmul) then runtime
+ detection isn't used and the generic code is omitted.
+
--enable-unaligned-access
Allow liblzma to use unaligned memory access for 16-bit,
32-bit, and 64-bit loads and stores. This should be