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arm: intreadwrite: fix inline asm constraints for gcc 4.6 and later

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With a dereferenced type-cast pointer as memory operand, gcc 4.6
and later will sometimes copy the data to a temporary location,
the address of which is used as the operand value, if it thinks
the target address might be misaligned.  Using a pointer to a
packed struct type instead does the right thing.

The 16-bit case is special since the ldrh instruction addressing
modes are limited compared to ldr.  The "Uq" constraint produces a
memory reference suitable for an ldrsb instruction, which supports
the same addressing modes as ldrh.  However, the restrictions appear
to apply only when the operand addresses a single byte.  The memory
reference must thus be split into two operands each targeting one
byte.  Finally, the "Uq" constraint is only available in ARM mode.
The Thumb-2 ldrh instruction supports most addressing modes so the
normal "m" constraint can be used there.

Signed-off-by: Mans Rullgard <mans@mansr.com>
This commit is contained in:
Mans Rullgard 2012-05-02 12:46:22 +01:00
parent 4b7fa553a9
commit adebad07e0
1 changed files with 10 additions and 3 deletions
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13
libavutil/arm/intreadwrite.h
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@ -27,8 +27,13 @@
#define AV_RN16 AV_RN16
static av_always_inline unsigned AV_RN16(const void *p)
{
const uint8_t *q = p;
unsigned v;
__asm__ ("ldrh %0, %1" : "=r"(v) : "m"(*(const uint16_t *)p));
#ifdef __thumb__
__asm__ ("ldrh %0, %1" : "=r"(v) : "m"(q[0]), "m"(q[1]));
#else
__asm__ ("ldrh %0, %1" : "=r"(v) : "Uq"(q[0]), "m"(q[1]));
#endif
return v;
}
@ -41,8 +46,9 @@ static av_always_inline void AV_WN16(void *p, uint16_t v)
#define AV_RN32 AV_RN32
static av_always_inline uint32_t AV_RN32(const void *p)
{
const struct __attribute__((packed)) { uint32_t v; } *q = p;
uint32_t v;
__asm__ ("ldr %0, %1" : "=r"(v) : "m"(*(const uint32_t *)p));
__asm__ ("ldr %0, %1" : "=r"(v) : "m"(*q));
return v;
}
@ -55,11 +61,12 @@ static av_always_inline void AV_WN32(void *p, uint32_t v)
#define AV_RN64 AV_RN64
static av_always_inline uint64_t AV_RN64(const void *p)
{
const struct __attribute__((packed)) { uint32_t v; } *q = p;
uint64_t v;
__asm__ ("ldr %Q0, %1 \n\t"
"ldr %R0, %2 \n\t"
: "=&r"(v)
: "m"(*(const uint32_t*)p), "m"(*((const uint32_t*)p+1)));
: "m"(q[0]), "m"(q[1]));
return v;
}