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

Sat May 28 01:37:45 PDT 2016

Instead of suppressing the integer promotion rules which are part of the ISO C/C++ Standards, we wrote a new pass that analyses the IR to see if the input values and output value were of an integer type that was narrower than the promoted types used in the IR, and if we could prove that the outcome would be identical if the type was unpromoted, then we reduced the IR to use the narrower form.

In our case the motive was to enhance vectorisation because our vector ALU can work with 8-, 16- and 32-bit integers natively, and handling ‘vXi8’ vectors ended was actually being promoted to multiple ‘v4i32’ vectors requiring 4 times as many instructions as were necessary, or worse still, fully scalarized.

This pass was presented by my colleague Stephen Rogers in a “Lighting Talk” at the October 2015 LLVM Conference in San Jose and titled “Integer Vector Optimizations and “Usual Arithmetic Conversions””.  I can’t find the paper or slides on the LLVM Meetings page, perhaps these are not archived for Lightning Talks (?), but as they are not large I have attached them here.

This approach allowed us to gain the optimisations that are possible with our architecture which supports 8-, 16- and 32-bit native integer computations (scalar and vector), while also respecting the ISO C and C++ Standards.  I am a lot more nervous of a front-end switch for this, as it will lead to non-compliant programs, and in the presence of overloading and template-instantiation it could also lead to very different programs, and would recommend that we do not add a front-end switch which alters the semantics of the language in this way.

It is my intention to publish this pass if it is of general interest, and since it is target independent there are no particular blocking issue for me (Patents, IP, etc.) to doing so.  I do have to catch-up on the HEAD revision to ensure that it still works correctly, but it was working perfectly at SVN #262824 and it will be a month before I have enough time to catch up on the HEAD revision as we are busy with a product release that takes precedence.

All the best,

            MartinO

From: cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] On Behalf Of David Majnemer via cfe-dev
Sent: 27 May 2016 19:55
To: Dilan Manatunga <manatunga at gmail.com>
Cc: clang developer list <cfe-dev at lists.llvm.org>; Norman Rink <norman.rink at tu-dresden.de>; cfe-dev-request at lists.llvm.org
Subject: Re: [cfe-dev] Disable integer promotion (Dilan Manatunga via cfe-dev)

You could set IntWidth to 16 or 8 in clang, not unlike what MSP430 does:

https://github.com/llvm-mirror/clang/blob/3317d0fa0bd1f5c5adc14bcc6adc2a38acc9064b/lib/Basic/Targets.cpp#L6823

On Fri, May 27, 2016 at 10:32 AM, Dilan Manatunga via cfe-dev <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org> > wrote:

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 <mailto: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 <mailto:cfe-dev at lists.llvm.org> >
>To: cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org> 
>Subject: [cfe-dev] Disable integer promotion
>Message-ID:
>       <CAHpgGu4=jFC9ohQQZZMp2NMG3Hw0sE5U4-Lqrgb+6gcXv9SEtQ at mail.gmail.com <mailto:jFC9ohQQZZMp2NMG3Hw0sE5U4-Lqrgb%2B6gcXv9SEtQ 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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