[Info-vax] x86-64 VMS executable image sizes and memory requirements ?

[John Reagan]

On Saturday, December 21, 2019 at 4:30:27 PM UTC-5, osuv... at gmail.com wrote:
> On Saturday, December 21, 2019 at 11:16:59 AM UTC-5, already... at yahoo.com wrote:
> > Few numbers for clang/LLVM on x86-64 (not VMS, of course)
> > 
> > Option .text size
> > -O0 75456
> > -Oz 45772
> > -Os 44992
> > -O1 56836
> > -O2 45220
> > -O3 46084
> > 
> 
> Clearly, DEC C isn't clang. Building the sqlite3shr shareable image on alpha
> with different /optimization levels gives (size in disk blocks):
> 
>   sqlite3shr-O0.exe;1             2676
>   sqlite3shr-O1.exe;1             2405
>   sqlite3shr-O2.exe;1             3513
>   sqlite3shr-O3.exe;1             3710
>   sqlite3shr-O4.exe;1             3710
>   sqlite3shr-O5.exe;1             3820
> 
> If I link /notrace, however, the file sizes change to:
>   sqlite3shr-O0.exe;1             2099
>   sqlite3shr-O1.exe;1             1835
>   sqlite3shr-O2.exe;1             1837
>   sqlite3shr-O3.exe;1             1962
>   sqlite3shr-O4.exe;1             1962
>   sqlite3shr-O5.exe;1             2059
> 
> I presume line number tracking is a lot more complicated for optimized code.
> 
> The respective $CODE$ sizes are:
>   sqlite3shr-O0.map;1           880492
>   sqlite3shr-O1.map;1           749156
>   sqlite3shr-O2.map;1           773240
>   sqlite3shr-O3.map;1           837496
>   sqlite3shr-O4.map;1           837496
>   sqlite3shr-O5.map;1           886824

Alpha doesn't have the complex addressing modes like x86 does.  Optimizations on Alpha tend to be code motions to hoist out of loop, strength reduction to avoid multiplies, common sub-expressions, etc.  Most of which have little impact on code size, just speed.  And at O2, routine inlining kicks in which is the size increase you see there.  You can control that with various keywords on /OPTIMIZE.

On x86, you'll see all sorts of clever addressing mode tricks from clang.

For exampile, I just tried a "* 15" with -Ofast with clang and got

int square(int num) {
    return num * 15;
}

square(int):                             # @square(int)
        lea     eax, [rdi + 4*rdi]
        lea     eax, [rax + 2*rax]
        ret

This is the kind of stuff you'd see in VAX BLISS or VAX Pascal.  Note, it isn't much smaller, just faster.