llvm dev - Dec 2018 - Where's the optimiser gone? (part 5.a): missed tail calls, and more...

Compile the following functions with "-O3 -target amd64"
(see <https://godbolt.org/z/5xqYhH>):

__int128 div(__int128 foo, __int128 bar)
{
    return foo / bar;
}

On the left the generated code; on the right the expected,
properly optimised code:

div: # @div
    push  rbp         |
    mov   rbp, rsp    |
    call  __divti3    |    jmp   __divti3
    pop   rbp         |
    ret               |


__int128 mod(__int128 foo, __int128 bar)
{
    return foo % bar;
}

mod: # @mod
    push  rbp         |
    mov   rbp, rsp    |
    call  __modti3    |    jmp   __modti3
    pop   rbp         |
    ret               |


__int128 mul(__int128 foo, __int128 bar)
{
    return foo * bar;
}

mul: # @mul
    push  rbp
    mov   rbp, rsp
    mov   r8, rdx
    mov   rax, rdx
    mul   rdi
    imul  rsi, r8
    add   rdx, rsi
    imul  rcx, rdi
    add   rdx, rcx
    pop   rbp
    ret

Hi,

On Sat, 1 Dec 2018 at 17:37, Stefan Kanthak via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Compile the following functions with "-O3 -target amd64"
You've been advised before, but you really need to start reporting
these as bugs[*] if you actually care about them. llvm-dev is
basically a black-hole as far as bugs go: if no-one takes enough of an
interest to either fix or file a bug within a day or two, the issue
will be completely forgotten. The list is intended for discussion of
issues and future designs, not reporting them.

It seems you got lucky and someone did file at least one report from
one of these threads; you should expect that to become less frequent.
People are willing to assume the best of new contributors, but by the
time you reach a dozen threads without actually engaging with the
community's procedures that goodwill starts to look a bit threadbare.

Cheers.

Tim.

[*] https://llvm.org/docs/Contributing.html

"Tim Northover" <t.p.northover at gmail.com> wrote:
> Hi,
> 
> On Sat, 1 Dec 2018 at 17:37, Stefan Kanthak via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> Compile the following functions with "-O3 -target amd64"
> 
> You've been advised before, but you really need to start reporting
> these as bugs[*] if you actually care about them.
These ain't my bugs, these are LLVM's bugs.
I don't use LLVM, I just dared to write some posts which pinpoint
deficiencies and bugs in it. You should see this as COURTESY.
The least I expect from any software developer, and especially a
"foundation", is to take action on bug reports, INDEPENDENT how they
are reported.
> llvm-dev is
> basically a black-hole as far as bugs go: if no-one takes enough of an
> interest to either fix or file a bug within a day or two, the issue
> will be completely forgotten. The list is intended for discussion of
> issues and future designs, not reporting them.
That's clearly LLVM's problem.
> It seems you got lucky and someone did file at least one report from
> one of these threads;
You got that backwards: YOU are lucky that I took the time to report
these bugs!
> you should expect that to become less frequent.
> People are willing to assume the best of new contributors, but by the
> time you reach a dozen threads without actually engaging with the
> community's procedures that goodwill starts to look a bit threadbare.
> 
> Cheers.
> 
> Tim.
> 
> [*] https://llvm.org/docs/Contributing.html
| If you are working with LLVM and run into a bug, we definitely want
| to know about it.

I DONT WORK WITH LLVM.
I was just curious whether it shows bugs and deficiencies I saw in
two other C compilers, and documented them.
You and your users suffer from these bugs and deficiencies, not me.

regards
Stefan