Given,
struct shared_ptr_struct
{
unsigned int phase : 24;
unsigned short thread : 16;
void *addr;
};
On the x86_64 (ie, Opteron[tm]) platform, GCC appears to designate the
underlying mode of this type as a BLKmode, instead of a TImode. This
has implications in terms of the quality of the code that is generated
to copy and manipulate 128 bit structures (as defined in the example
above).
The decision to commit this type to a BLKmode value, originates
in this logic in mode_for_size():
if (limit && size > MAX_FIXED_MODE_SIZE)
return BLKmode;
On the x86 platform, there appears to be no target definition for
MAX_FIELD_SIZE. Thus, the default in stor-layout.c applies:
#ifndef MAX_FIXED_MODE_SIZE
#define MAX_FIXED_MODE_SIZE GET_MODE_BITSIZE (DImode)
#endif
Other 64 bit targets define MAX_FIXED_MODE_SIZE along these lines
(some line wrapping may occur below):
config/i960/i960.h:#define MAX_FIXED_MODE_SIZE GET_MODE_BITSIZE
(TImode)
config/ia64/ia64.h:#define MAX_FIXED_MODE_SIZE GET_MODE_BITSIZE (TImode)
config/mips/mips.h:#define MAX_FIXED_MODE_SIZE LONG_DOUBLE_TYPE_SIZE
config/sh/sh.h:#define MAX_FIXED_MODE_SIZE (TARGET_SH5 ? 128 : 64)
on MIPS, LONG_DOUBLE_TYPE_SIZE is defined as follows:
/* A C expression for the size in bits of the type `long double' on
the target machine. If you don't define this, the default is two
words. */
#define LONG_DOUBLE_TYPE_SIZE \
(mips_abi == ABI_N32 || mips_abi == ABI_64 ? 128 : 64)
In the 'dev' tree, the s390 defines MAX_FIXED_MODE_SIZE as follows:
config/s390/s390.h:#define MAX_FIXED_MODE_SIZE GET_MODE_BITSIZE
(TARGET_64BIT ? TImode : DImode)
(Arguably, the s390 variant might be a better default value to be defined
in stor-layout.c)
I haven't tried making the suggested change to see if the x86_64 code
generator can fully support it. Are there any technical reasons that
the x86_64 shouldn't target 128 bit structs into a long double (ie, two
64 bit registers)?
