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libfdt: Don't use memcpy to handle unaligned reads on ARM

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6dcb8ba4 "libfdt: Add helpers for accessing unaligned words" introduced
the fdt32_ld() and fdt64_ld() helpers for loading values from the FDT blob
which might not be naturally aligned.  This matters for ARM, where
attempting a plain unaligned load will often cause an exception.

However, it seems the memcpy() we used here was surprisingly expensive,
making libfdt nearly 6x slower on at least some ARM platforms.

This patch takes an alternative approach, using a bunch of 1-byte loads
and shifts to implement the helpers.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
This commit is contained in:
David Gibson 2018-11-19 16:36:15 +11:00
parent 86a288a736
commit 11738cf01f
1 changed files with 14 additions and 6 deletions
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20
libfdt/libfdt.h
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@ -163,18 +163,26 @@ uint32_t fdt_next_tag(const void *fdt, int offset, int *nextoffset);
static inline uint32_t fdt32_ld(const fdt32_t *p)
{
fdt32_t v;
const uint8_t *bp = (const uint8_t *)p;
memcpy(&v, p, sizeof(v));
return fdt32_to_cpu(v);
return ((uint32_t)bp[0] << 24)
| ((uint32_t)bp[1] << 16)
| ((uint32_t)bp[2] << 8)
| bp[3];
}
static inline uint64_t fdt64_ld(const fdt64_t *p)
{
fdt64_t v;
const uint8_t *bp = (const uint8_t *)p;
memcpy(&v, p, sizeof(v));
return fdt64_to_cpu(v);
return ((uint64_t)bp[0] << 56)
| ((uint64_t)bp[1] << 48)
| ((uint64_t)bp[2] << 40)
| ((uint64_t)bp[3] << 32)
| ((uint64_t)bp[4] << 24)
| ((uint64_t)bp[5] << 16)
| ((uint64_t)bp[6] << 8)
| bp[7];
}
/**********************************************************************/