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Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
This commit is contained in:
Dave Rodgman 2024-02-06 13:51:58 +00:00
parent 22b934e6d2
commit d09f96b829
1 changed files with 11 additions and 5 deletions
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16
library/alignment.h
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@ -58,12 +58,15 @@ typedef uint64_t __packed mbedtls_uint64_unaligned_t;
* gcc may generate a branch to memcpy for calls like `memcpy(dest, src, 4)` rather than * gcc may generate a branch to memcpy for calls like `memcpy(dest, src, 4)` rather than
* generating some LDR or LDRB instructions (similar for stores). * generating some LDR or LDRB instructions (similar for stores).
* *
* For versions of gcc < 5.4.0 this always happens. * This is architecture dependent: x86-64 seems fine even with old gcc; 32-bit Arm
* For gcc < 6.3.0, this happens at -O0 * is affected. To keep it simple, we enable for all architectures.
* For all versions, this happens iff unaligned access is not supported.
* *
* For gcc 4.x, this will generate byte-by-byte loads even if unaligned access is supported, which * For versions of gcc < 5.4.0 this issue always happens.
* is correct but not optimal. * For gcc < 6.3.0, this issue happens at -O0
* For all versions, this issue happens iff unaligned access is not supported.
*
* For gcc 4.x, this implementation will generate byte-by-byte loads even if unaligned access is
* supported, which is correct but not optimal.
* *
* For performance (and code size, in some cases), we want to avoid the branch and just generate * For performance (and code size, in some cases), we want to avoid the branch and just generate
* some inline load/store instructions since the access is small and constant-size. * some inline load/store instructions since the access is small and constant-size.
@ -73,6 +76,9 @@ typedef uint64_t __packed mbedtls_uint64_unaligned_t;
* possible alignment—one byte for a variable" * possible alignment—one byte for a variable"
* https://gcc.gnu.org/onlinedocs/gcc-4.5.4/gcc/Variable-Attributes.html * https://gcc.gnu.org/onlinedocs/gcc-4.5.4/gcc/Variable-Attributes.html
* *
* Previous implementations used __attribute__((__aligned__(1)), but had issues with a gcc bug:
* https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94662
*
* Tested with several versions of GCC from 4.5.0 up to 13.2.0 * Tested with several versions of GCC from 4.5.0 up to 13.2.0
* We don't enable for older than 4.5.0 as this has not been tested. * We don't enable for older than 4.5.0 as this has not been tested.
*/ */