llvm dev - Aug 2015 - Support for byte sizes larger than 8 bits

Hello LLVMdev,

lately the company I work for has bumped into a need for a way to 
specify a size of byte larger than the regular 8 bits for our targets. 
After a bit of research we found out that the feature would require 
changes across most of the LLVM framework (including clang) and 
maintaining such changes without involving the LLVM devs would be 
tedious to say the least.

So my first question would be if such a feature would be welcome in the 
LLVM framework. The motivation for implementing the feature is that 
certain embedded processors often use custom byte sizes, such as 12 
bits, to conserve power while addressing its memory.

The first solution sketch we came up with is to introduce a new field 
into the data layout string that would specify the size of a byte (or 
char). The default value of the field would be the regular 8 bits. This 
way, the information should be propagated through all the necessary 
parts of LLVM.

The second issue is how to find and modify the existing code that relies 
on the assumption that a byte (or char) in the target is always 8 bits.

The changes we would introduce, should the feature be accepted, would of 
course be tested against an existing LLVM target and test-suites to 
ensure that we didn't break anything.

Regards,

Marek Surovic
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We maintain a set of patches, supporting 16-bit bytes for LLVM (currently not
Clang), which may be provided upon request.

/Patrik Hägglund

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Marek
Surovic via llvm-dev
Sent: den 17 augusti 2015 15:48
To: llvm-dev at lists.llvm.org
Subject: [llvm-dev] Support for byte sizes larger than 8 bits

Hello LLVMdev,

lately the company I work for has bumped into a need for a way to specify a size
of byte larger than the regular 8 bits for our targets. After a bit of research
we found out that the feature would require changes across most of the LLVM
framework (including clang) and maintaining such changes without involving the
LLVM devs would be tedious to say the least.

So my first question would be if such a feature would be welcome in the LLVM
framework. The motivation for implementing the feature is that certain embedded
processors often use custom byte sizes, such as 12 bits, to conserve power while
addressing its memory.

The first solution sketch we came up with is to introduce a new field into the
data layout string that would specify the size of a byte (or char). The default
value of the field would be the regular 8 bits. This way, the information should
be propagated through all the necessary parts of LLVM.

The second issue is how to find and modify the existing code that relies on the
assumption that a byte (or char) in the target is always 8 bits.

The changes we would introduce, should the feature be accepted, would of course
be tested against an existing LLVM target and test-suites to ensure that we
didn't break anything.

Regards,

Marek Surovic
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