[cfe-dev] Disable integer promotion (Dilan Manatunga via cfe-dev)

[Dilan Manatunga via cfe-dev]

I need disabling this feature because I am researching architectures where
8-bit or 16-bit adds are preferred to 32-bit. So, integer promotion kinda
mucks everything up. I was hoping there was a way in clang to disable it,
instead of having to implement an LLVM pass to coalesce unnecessary
promotions.

Thanks for catching the IR mistake. Should have double checked that. This
should be the correct version:
nt8_t a  = 1;
int8_t b = 2;
int8_t c = a + b

The LLVM IR will be:
%x = sext i8 %a to i32
%y = sext i8 %b to i32
%z = add nsw i32 %x, %y
%c = trunc i32 %z to i8

Instead, it would simply compile to:
$c = add nsw i8 %z, $y

-Dilan


On Fri, May 27, 2016 at 5:30 AM Norman Rink via cfe-dev <
cfe-dev at lists.llvm.org> wrote:

> Hi Dilan,
>
> I would like to second your request for an option to disable integer
> promotion. What do you need it for?
>
> As far as I am aware, there is no such option and the code that implements
> integer promotion is somewhat scattered across ³SemaExpr.cpp².
>
> Also, I think your example code snippet contains a few ³i32²s too many. It
> will be clearer to people what you are looking for if your code example is
> consistent with your question.
>
> Best,
>
> Norman
>
>
> >Message: 1
> >Date: Fri, 27 May 2016 01:50:12 +0000
> >From: Dilan Manatunga via cfe-dev <cfe-dev at lists.llvm.org>
> >To: cfe-dev at lists.llvm.org
> >Subject: [cfe-dev] Disable integer promotion
> >Message-ID:
> >       <CAHpgGu4=
> jFC9ohQQZZMp2NMG3Hw0sE5U4-Lqrgb+6gcXv9SEtQ at mail.gmail.com>
> >Content-Type: text/plain; charset="utf-8"
> >
> >Is there a way to disable integer promotion when performing math
> >operations. For example, when compiling a statement such as this:
> >int8_t a  = 1;
> >int8_t b = 2;
> >int8_t c = a + b
> >
> >The LLVM IR will be:
> >%x = sext i32 %a to i32
> >%y = sext i32 %b to i32
> >%z = add nsw i32 %x, %y
> >%c = trunc i32 %z to i16
> >
> >Instead, it would simply compile to:
> >$c = add nsw i32 %z, $y
> >
> >-Dilan Manatunga
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