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Signed-off-by: Dave Rodgman <dave.rodgman@arm.com>
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Dave Rodgman
2024-02-06 12:57:03 +00:00
parent ec9936d122
commit f4e8234f93
1 changed files with 13 additions and 8 deletions
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21
library/alignment.h
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@@ -55,20 +55,25 @@ typedef uint64_t __packed mbedtls_uint64_unaligned_t;
#elif defined(MBEDTLS_COMPILER_IS_GCC) && (MBEDTLS_GCC_VERSION >= 40504) && \
((MBEDTLS_GCC_VERSION < 60300) || (!defined(MBEDTLS_EFFICIENT_UNALIGNED_ACCESS)))
/*
* Old versions of gcc, depending on how the target is specified, may generate a branch to memcpy
* for calls like `memcpy(dest, src, 4)` rather than generating some LDR or LDRB instructions
* (similar for stores).
* Recent versions where unaligned access is not enabled also do this.
* 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).
*
* For versions of gcc < 5.4.0 this always happens.
* For gcc < 6.3.0, this happens at -O0
* 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
* is correct but not optimal.
*
* 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.
*
* The manual states:
* "The aligned attribute specifies a minimum alignment for the variable or structure field,
* measured in bytes."
* https://gcc.gnu.org/onlinedocs/gcc/Common-Variable-Attributes.html
* "The packed attribute specifies that a variable or structure field should have the smallest
* possible alignment—one byte for a variable"
* https://gcc.gnu.org/onlinedocs/gcc-4.5.4/gcc/Variable-Attributes.html
*
* Tested with several versions of GCC from 4.5.0 up to 9.3.0
* 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.
*/
#define UINT_UNALIGNED_UNION