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softfloat: use GCC builtins to count the leading zeros 

Softfloat has its own implementation to count the leading zeros. However
a lot of architectures have either a dedicated instruction or an
optimized to do that. When using GCC >= 3.4, this patch uses GCC builtins
instead of the handcoded implementation.

Note that I amware that QEMU_GNUC_PREREQ is defined in osdep.h and that
clz32() and clz64() are defined in host-utils.h, but I think it is better
to keep the softfloat implementation self contained.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
This commit is contained in:
Aurelien Jarno 2011-04-14 00:49:29 +02:00
parent a0102082de
commit b3b4c7f33f
1 changed files with 27 additions and 2 deletions
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29
fpu/softfloat-macros.h
View File

@ -35,6 +35,17 @@ these four paragraphs for those parts of this code that are retained.
=============================================================================*/
/*----------------------------------------------------------------------------
| This macro tests for minimum version of the GNU C compiler.
*----------------------------------------------------------------------------*/
#if defined(__GNUC__) && defined(__GNUC_MINOR__)
# define SOFTFLOAT_GNUC_PREREQ(maj, min) \
((__GNUC__ << 16) + __GNUC_MINOR__ >= ((maj) << 16) + (min))
#else
# define SOFTFLOAT_GNUC_PREREQ(maj, min) 0
#endif
/*----------------------------------------------------------------------------
| Shifts `a' right by the number of bits given in `count'. If any nonzero
| bits are shifted off, they are ``jammed'' into the least significant bit of
@ -616,6 +627,13 @@ static uint32_t estimateSqrt32( int16 aExp, uint32_t a )
static int8 countLeadingZeros32( uint32_t a )
{
#if SOFTFLOAT_GNUC_PREREQ(3, 4)
if (a) {
return __builtin_clz(a);
} else {
return 32;
}
#else
static const int8 countLeadingZerosHigh[] = {
8, 7, 6, 6, 5, 5, 5, 5, 4, 4, 4, 4, 4, 4, 4, 4,
3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3, 3,
@ -647,7 +665,7 @@ static int8 countLeadingZeros32( uint32_t a )
}
shiftCount += countLeadingZerosHigh[ a>>24 ];
return shiftCount;
#endif
}
/*----------------------------------------------------------------------------
@ -657,6 +675,13 @@ static int8 countLeadingZeros32( uint32_t a )
static int8 countLeadingZeros64( uint64_t a )
{
#if SOFTFLOAT_GNUC_PREREQ(3, 4)
if (a) {
return __builtin_clzll(a);
} else {
return 64;
}
#else
int8 shiftCount;
shiftCount = 0;
@ -668,7 +693,7 @@ static int8 countLeadingZeros64( uint64_t a )
}
shiftCount += countLeadingZeros32( a );
return shiftCount;
#endif
}
/*----------------------------------------------------------------------------