Re: [HACKERS] Use static inline functions for Float <-> Datum conversions
Heikki Linnakangas writes:
> On 08/31/2016 02:38 PM, Tom Lane wrote:
>> I wonder whether there is a compiler-dependent way of avoiding the union
>> trick ... or maybe gcc is already smart enough that it doesn't matter?
> It seems to compile into a single instruction, so it can't get any
> better from a performance point of view.
Yeah, confirmed here. On my not-real-new gcc (version 4.4.7, which
ships with RHEL6), these test functions:
Datum
compare_int8(PG_FUNCTION_ARGS)
{
int64 x = PG_GETARG_INT64(0);
int64 y = PG_GETARG_INT64(1);
PG_RETURN_BOOL(x < y);
}
Datum
compare_float8(PG_FUNCTION_ARGS)
{
double x = PG_GETARG_FLOAT8(0);
double y = PG_GETARG_FLOAT8(1);
PG_RETURN_BOOL(x < y);
}
compile into this (at -O2):
compare_int8:
.cfi_startproc
movq40(%rdi), %rax
cmpq%rax, 32(%rdi)
setl%al
movzbl %al, %eax
ret
.cfi_endproc
compare_float8:
.cfi_startproc
movsd 40(%rdi), %xmm0
xorl%eax, %eax
ucomisd 32(%rdi), %xmm0
seta%al
ret
.cfi_endproc
(Not sure why the compiler does the widening of the comparison result
differently, but it doesn't look like it matters.) Before this patch,
that looked like:
compare_float8:
.cfi_startproc
pushq %rbx
.cfi_def_cfa_offset 16
.cfi_offset 3, -16
movq%rdi, %rbx
subq$16, %rsp
.cfi_def_cfa_offset 32
movq32(%rdi), %rdi
callDatumGetFloat8
movq40(%rbx), %rdi
movsd %xmm0, 8(%rsp)
callDatumGetFloat8
xorl%eax, %eax
ucomisd 8(%rsp), %xmm0
seta%al
addq$16, %rsp
.cfi_def_cfa_offset 16
popq%rbx
.cfi_def_cfa_offset 8
ret
.cfi_endproc
Nice.
regards, tom lane
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Re: [HACKERS] Use static inline functions for Float <-> Datum conversions
On 08/31/2016 02:38 PM, Tom Lane wrote: Heikki Linnakangas writes: Now that we are OK with static inline functions, we can save some cycles from floating-point functions, by turning Float4GetDatum, Float8GetDatum, and DatumGetFloat8 into static inlines. Looks good to me. Ok, will push. I wonder whether there is a compiler-dependent way of avoiding the union trick ... or maybe gcc is already smart enough that it doesn't matter? It seems to compile into a single instruction, so it can't get any better from a performance point of view. float8pl: .LFB79: .loc 1 871 0 .cfi_startproc .LVL297: .LBB959: .LBB960: .loc 2 733 0 movsd 40(%rdi), %xmm2 .LBE960: .LBE959: .LBB961: .LBB962: movsd 32(%rdi), %xmm1 ... A union is probably what language pedantics would prefer anyway, and anything else would be more of a trick. - Heikki -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
Re: [HACKERS] Use static inline functions for Float <-> Datum conversions
Heikki Linnakangas writes: > Now that we are OK with static inline functions, we can save some cycles > from floating-point functions, by turning Float4GetDatum, > Float8GetDatum, and DatumGetFloat8 into static inlines. Looks good to me. I wonder whether there is a compiler-dependent way of avoiding the union trick ... or maybe gcc is already smart enough that it doesn't matter? regards, tom lane -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
