llvm dev - Jan 2015 - [LLVMdev] CPUStringIsValid() into MCSubtargetInfo and use it for ARM .cpu parsing

Hi Divacky,

  I have an armv7 variant that supports hardware division (extension).

For my variant, I use ".cpu cortex-a9" and division
attribute(.eabi_attribute 44, 2   @ Tag_DIV_use) to let the assembler do the
right thing if it encounters a division instruction.

With your path, the .cpu directive is used to fetch the available features
of a CPU and ignores the eabi attributes.

What should be the approach to fix the issue which I am facing ?

Shouldn't the eabi attributes be honored ?

 

--Sumanth G

 

 

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Hi Sumanth,
> Shouldn’t the eabi attributes be honored ?
EABI attributes are instructions for the linker on which object files
are compatible with each other, not instructions for the assembler
(and trying to make them so sounds like a recipe for confusion to
me).
>From the ARM ABI addenda:
"Build attributes record data that a linker needs to reason
mechanically about the compatibility, or incompatibility, of a set of
relocatable files."
> What should be the approach to fix the issue which I am facing?
There are a few options here:
1. Specify an existing CPU that has hardware division (notably,
Cortex-A9 doesn't).
2. Teach LLVM & Clang about your CPU's features, either locally or
upstream.
3. Pass "-mhwdiv=arm,thumb" to Clang (or less if you only have hwdiv
in one mode).

Cheers.

Tim.

Tim,
       How about the below option ?

1. Specify an existing generic armv7 CPU or the CPU which is close my custom
variant. My custom variant can be treated as "cortex-a9" + hwdiv.
    So my CPU here is "cortex-a9"
2. Specify the ".arch_extension idiv" which is available as an
extension for my custom variant.
3. Teach LLVM & Clang about your CPU's features, either locally or
upstream.
4. Pass "-mhwdiv=arm,thumb" to Clang (or less if you only have hwdiv
in one mode).

--Sumanth G
-----Original Message-----
From: Tim Northover [mailto:t.p.northover at gmail.com] 
Sent: Tuesday, January 27, 2015 6:44 PM
To: Sumanth Gundapaneni
Cc: LLVM Developers Mailing List; Roman Divacky; Ana Pazos
Subject: Re: [LLVMdev] CPUStringIsValid() into MCSubtargetInfo and use it for
ARM .cpu parsing

Hi Sumanth,
> Shouldn’t the eabi attributes be honored ?
EABI attributes are instructions for the linker on which object files are
compatible with each other, not instructions for the assembler (and trying to
make them so sounds like a recipe for confusion to me).
>From the ARM ABI addenda:
"Build attributes record data that a linker needs to reason mechanically
about the compatibility, or incompatibility, of a set of relocatable
files."
> What should be the approach to fix the issue which I am facing?
There are a few options here:
1. Specify an existing CPU that has hardware division (notably,
Cortex-A9 doesn't).
2. Teach LLVM & Clang about your CPU's features, either locally or
upstream.
3. Pass "-mhwdiv=arm,thumb" to Clang (or less if you only have hwdiv
in one mode).

Cheers.

Tim.