llvm dev - Nov 2012 - [LLVMdev] Set the minimum number of allocated bits for a variable

Hi,

I would like to force the minimum number of bits allocated for a variable in
memory to be 16. From what I have seen, i1 is already represented on 8 bits. So,
the only change would be to represent i1 and i8 on 16 bits, as all other types
already fulfill this condition.

TargetData can help by setting a higher alignment, thus although the type is i1
or i8, the number of allocated bits is 16.

What is the best approach to impose this?

(i) modifying the back-end of LLVM 
    - where can I find the defaults for specifying the number of allocated bits
for each type?
(ii) writing a pass that overwrites these defaults?
   - by asking for an alignment higher than the default

   - one approach is to set a higher alignment for each allocated variable of
type i1 or i8, but this means digging inside the arrays, vectors and structures
to check the type of each element.

 
Looking forward to receiving your suggestions.


Thank you,
Alexandra
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Hi Alexandra,

I'm not sure want you want to do. Is the data layout string
(http://llvm.org/docs/LangRef.html#datalayout, also usually set by each target
specific *TargetMachine constructor), for setting the alignment good enough for
you?

Or is it more than the alignment you want to control? Do you have a target with
16-bit bytes? In that case, there is quite a lot of changes that needs to be
done (we maintain some patches internally that we can provide upon request).

Regards,
Patrik Hägglund

From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Jimborean Alexandra
Sent: den 22 november 2012 13:07
To: llvmdev at cs.uiuc.edu
Subject: [LLVMdev] Set the minimum number of allocated bits for a variable

Hi,

I would like to force the minimum number of bits allocated for a variable in
memory to be 16. From what I have seen, i1 is already represented on 8 bits. So,
the only change would be to represent i1 and i8 on 16 bits, as all other types
already fulfill this condition.

TargetData can help by setting a higher alignment, thus although the type is i1
or i8, the number of allocated bits is 16.
What is the best approach to impose this?
(i) modifying the back-end of LLVM
    - where can I find the defaults for specifying the number of allocated bits
for each type?
(ii) writing a pass that overwrites these defaults?
   - by asking for an alignment higher than the default
   - one approach is to set a higher alignment for each allocated variable of
type i1 or i8, but this means digging inside the arrays, vectors and structures
to check the type of each element.

Looking forward to receiving your suggestions.

Thank you,
Alexandra
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Hello Patrik,

Thank you for your reply. Indeed, setting the alignment using the constructor is
sufficient.


Regards,

Alexandra



________________________________
 From: Patrik Hägglund H <patrik.h.hagglund at ericsson.com>
To: Jimborean Alexandra <xinfinity_a at yahoo.com> 
Cc: "llvmdev at cs.uiuc.edu" <llvmdev at cs.uiuc.edu> 
Sent: Thursday, November 22, 2012 1:47 PM
Subject: RE: [LLVMdev] Set the minimum number of allocated bits for a variable
 

 
Hi Alexandra,
 
I’m not sure want you want to do. Is the data layout string
(http://llvm.org/docs/LangRef.html#datalayout, also usually set by each target
specific *TargetMachine constructor), for setting the alignment good enough for
you?
 
Or is it more than the alignment you want to control? Do you have a target with
16-bit bytes? In that case, there is quite a lot of changes that needs to be
done (we maintain some patches internally that we can provide upon request).
 
Regards,
Patrik Hägglund
 
From:llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Jimborean Alexandra
Sent: den 22 november 2012 13:07
To: llvmdev at cs.uiuc.edu
Subject: [LLVMdev] Set the minimum number of allocated bits for a variable
 
Hi,
 
I would like to force the minimum number of bits allocated for a variable in
memory to be 16. From what I have seen, i1 is already represented on 8 bits. So,
the only change would be to represent i1 and i8 on 16 bits, as all other types
already fulfill this condition.
 
TargetData can help by setting a higher alignment, thus although the type is i1
or i8, the number of allocated bits is 16.
What is the best approach to impose this?
(i) modifying the back-end of LLVM 
    - where can I find the defaults for specifying the number of allocated bits
for each type?
(ii) writing a pass that overwrites these defaults?
   - by asking for an alignment higher than the default
   - one approach is to set a higher alignment for each allocated variable of
type i1 or i8, but this means digging inside the arrays, vectors and structures
to check the type of each element.
 
Looking forward to receiving your suggestions.
 
Thank you,
Alexandra
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