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| author | Lasse Collin <lasse.collin@tukaani.org> | 2023-09-23 23:28:48 +0300 |
|---|---|---|
| committer | Lasse Collin <lasse.collin@tukaani.org> | 2023-10-18 19:02:45 +0300 |
| commit | 2f81ac852bc5aafc91c8e2adc66b5114761703c4 (patch) | |
| tree | 66d661327a0f0ab993c120dd67c5eb70c6f24e3c /INSTALL | |
| parent | tuklib_integer: Revise unaligned reads and writes on strict-align archs. (diff) | |
| download | xz-2f81ac852bc5aafc91c8e2adc66b5114761703c4.tar.xz | |
Build: Enabled unaligned access by default on PowerPC64LE and some RISC-V.
PowerPC64LE wasn't tested but it seems like a safe change.
POWER8 supports unaligned access in little endian mode. Testing
on godbolt.org shows that GCC uses unaligned access by default.
The RISC-V macro __riscv_misaligned_fast is very new and not
in any stable compiler release yet.
Documentation in INSTALL was updated to match.
Documentation about an autodetection bug when using ARM64 GCC
with -mstrict-align was added to INSTALL.
CMake files weren't updated yet.
Diffstat (limited to 'INSTALL')
| -rw-r--r-- | INSTALL | 39 |
1 files changed, 37 insertions, 2 deletions
@@ -400,8 +400,43 @@ XZ Utils Installation slow. This option shouldn't be used on systems that rely on such emulation. - Unaligned access is enabled by default on x86, x86-64, - big endian PowerPC, some ARM, and some ARM64 systems. + Unaligned access is enabled by default on these: + - 32-bit x86 + - 64-bit x86-64 + - 32-bit big endian PowerPC + - 64-bit big endian PowerPC + - 64-bit little endian PowerPC + - some RISC-V [1] + - some 32-bit ARM [2] + - some 64-bit ARM64 [2] (NOTE: Autodetection bug + if using GCC -mstrict-align, see below.) + + [1] Unaligned access is enabled by default if + configure sees that the C compiler + #defines __riscv_misaligned_fast. + + [2] Unaligned access is enabled by default if + configure sees that the C compiler + #defines __ARM_FEATURE_UNALIGNED: + + - ARMv7 + GCC or Clang: It works. The options + -munaligned-access and -mno-unaligned-access + affect this macro correctly. + + - ARM64 + Clang: It works. The options + -munaligned-access, -mno-unaligned-access, + and -mstrict-align affect this macro correctly. + Clang >= 17 supports -mno-strict-align too. + + - ARM64 + GCC: It partially works. The macro + is always #defined by GCC versions at least + up to 13.2, even when using -mstrict-align. + If building for strict-align ARM64, the + configure option --disable-unaligned-access + should be used if using a GCC version that has + this issue because otherwise the performance + may be degraded. It likely won't crash due to + how unaligned access is done in the C code. --enable-unsafe-type-punning This enables use of code like |