Re: [GENERAL] Is float8 a reference type? - Mailing list pgsql-general

On Fri, Sep 22, 2017 at 8:38 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> "Premature optimization is the root of all evil".  Do you have good reason
> to think that it's worth your time to write unsafe/unportable code?  Do
> you know that your compiler doesn't turn Float8GetDatum into a no-op
> already?  (Mine does, on a 64-bit machine.)

Ha ha, thank you for keeping me honest! But can you explain what is
unsafe about the cast? For a little more context: I've loaded a float8
array from a file, but I need to pass a Datum array to
construct_md_array. With an 8-byte Datum, I can just pass the original
float array, right? But with smaller Datums I need to go through the
array and convert each element. (I'm not really worried about these
files being moved between machines, so I'm willing to make the on-disk
format the same as the in-memory format.)

Since I'm expecting ~10 million elements per array, it seems like
skipping the conversion will have a real effect. I checked the
assembly and do see a difference (on both Mac+clang and Linux+gcc).
Here is the Mac command line:
   platter:floatfile paul$ clang -Wall -Wmissing-prototypes
-Wpointer-arith -Wdeclaration-after-statement -Wendif-labels
-Wmissing-format-attribute -Wformat-security -fno-strict-aliasing
-fwrapv -Wno-unused-command-line-argument -O2  -I. -I./
-I/usr/local/Cellar/postgresql@9.6/9.6.3/include/server
-I/usr/local/Cellar/postgresql@9.6/9.6.3/include/internal
-I/usr/local/opt/gettext/include -I/usr/local/opt/openldap/include
-I/usr/local/opt/openssl/include -I/usr/local/opt/readline/include
-I/usr/local/opt/tcl-tk/include -g -S -o floatfile.s floatfile.c

Here is the assembly for the cast:
     .loc    2 391 23 is_stmt 1      ## floatfile.c:391:23     movq    -48(%rbp), %r15   Ltmp176:     ##DEBUG_VALUE:
load_floatfile:datums<- %R15
 

Here is the assembly for the loop (after just changing the code to `if
(FLOAT8PASSBYVAL && false)`):
     .loc    2 393 21 is_stmt 1      ## floatfile.c:393:21     movslq    %r15d, %r13     .loc    2 393 28 is_stmt 0
## floatfile.c:393:28     leaq    (,%r13,8), %rdi     .loc    2 393 14                ## floatfile.c:393:14     callq
_palloc     movq    %rax, %r12   Ltmp177:     ##DEBUG_VALUE: load_floatfile:i <- 0     .loc    2 394 19 is_stmt 1
discriminator1 ## floatfile.c:394:19     testl    %r15d, %r15d   Ltmp178:     .loc    2 394 5 is_stmt 0 discriminator 1
##floatfile.c:394:5     je    LBB7_11   Ltmp179:   ## BB#9:     ##DEBUG_VALUE: load_floatfile:arrlen <- %R15D
##DEBUG_VALUE:load_floatfile:nulls <- [%RBP+-80]     ##DEBUG_VALUE: load_floatfile:floats <- [%RBP+-72]
##DEBUG_VALUE:load_floatfile:filename <- %RBX     .loc    2 0 5 discriminator 1   ## floatfile.c:0:5     movq
-72(%rbp),%rbx   Ltmp180:     ##DEBUG_VALUE: load_floatfile:floats <- %RBX     xorl    %r14d, %r14d   Ltmp181:
.p2align   4, 0x90   LBB7_10:                                ## =>This Inner Loop Header: Depth=1     ##DEBUG_VALUE:
load_floatfile:floats<- %RBX     ##DEBUG_VALUE: load_floatfile:arrlen <- %R15D     ##DEBUG_VALUE: load_floatfile:nulls
<-[%RBP+-80]     .loc    2 395 34 is_stmt 1      ## floatfile.c:395:34     movsd    (%rbx,%r14,8), %xmm0    ## xmm0 =
mem[0],zero    .loc    2 395 19 is_stmt 0      ## floatfile.c:395:19     callq    _Float8GetDatum     .loc    2 395 17
             ## floatfile.c:395:17     movq    %rax, (%r12,%r14,8)   Ltmp182:     .loc    2 394 30 is_stmt 1
discriminator2 ## floatfile.c:394:30     incq    %r14     .loc    2 394 19 is_stmt 0 discriminator 1 ##
floatfile.c:394:19    cmpq    %r13, %r14   Ltmp183:     .loc    2 394 5 discriminator 1 ## floatfile.c:394:5     jl
LBB7_10  Ltmp184:   LBB7_11:     ##DEBUG_VALUE: load_floatfile:arrlen <- %R15D     ##DEBUG_VALUE: load_floatfile:nulls
<-[%RBP+-80]
 

I get the same results on gcc too: the palloc, the loop, and even
`call Float8GetDatum@PLT`.

I'll do some timing of each version too, but it doesn't look like a
pointless optimization. I'd still like to know what is unsafe about it
though.

Thanks!
Paul


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