5.7.1.12 Libc-variant Option
The -mlibc-variant=variant option specifies whether speed- or
size-orientated string functions are linked in with the program. Size-orientated functions are
linked in if this option is not used.
Many of the library functions provided by
<string.h> are available in two variants. The size-orientated
functions require less code to implement. These can be explicitly selected by using a
size argument to the option, for example
-mlibc-variant=size. The speed-orientated functions might be larger, but
they execute quickly, resulting in improved program performance. They can be selected by using
a speed argument to the option.
Important: Note that some of the
speed-orientated functions might use the device's Floating-Point Unit (FPU) or Digital Signal
Processor (DSP) extension, when this is available. In some cases, the usage of the FPU or DSP
might require the interrupt context switching code to save and restore more registers, thereby
affecting the program's interrupt latency. Take this into consideration when calling these
string functions from an interrupt context.
All architectures implement the size-orientated function variants. The
table below shows those architectures that additionally support speed-orientated variants. In
the table, Single Precision Floating Point (FP), Double Precision Floating Point (DF), and
Single Instruction Multiple Data Parallel Processing (SIMD) architecture extensions are
listed. For cases where only the size-orientated variant is available, it will always be
selected, regardless of the variant specified by this option.
| Arm architecture | memset() | memcpy() | strlen() | strcmp() | memmove() | memchr() |
|---|---|---|---|---|---|---|
| ARMv6-M | ＋ speed | ＋ speed | ＋ speed | |||
| ARMv7E-M | ＋ speed | ＋ speed | ＋ speed | ＋ speed | ＋ speed | ＋ speed |
| ARMv7E-M+DP | ＋ speed | ＋ speed | ＋ speed | ＋ speed | ＋ speed | ＋ speed |
| ARMv7E-M+FP | ＋ speed | ＋ speed | ＋ speed | ＋ speed | ＋ speed | ＋ speed |
| ARMv7VE+SIMD | ＋ speed | ＋ speed | ＋ speed | ＋ speed | ＋ speed | |
| ARMv7-A | ＋ speed | ＋ speed | ＋ speed | ＋ speed | ＋ speed | |
| ARMv7-A+FP | ＋ speed | ＋ speed | ＋ speed | ＋ speed | ＋ speed | |
| ARMv7-A+SIMD | ＋ speed | ＋ speed | ＋ speed | ＋ speed | ＋ speed | |
| ARMv8-M Baseline | ＋ speed | ＋ speed | ＋ speed | |||
| ARMv8-M Mainline | ＋ speed | ＋ speed | ＋ speed | ＋ speed | ||
| ARMv8-M Mainline+FP | ＋ speed | ＋ speed | ＋ speed | ＋ speed |