writing a new Machine Description file (e.g cortex-a57.md) in gcc

[Virendra Kumar Pathak]

Hi Linaro Toolchain Group,

I am new to gcc development. I am trying to write a new md file describing
pipeline information for a processor.
Please suggest some good reference document for understanding machine
description file.

Few questions from cortex-a53.md file:

For first integer pipeline following is defined - (define_cpu_unit
"cortex_a53_slot0" "cortex_a53")

Is name cortex_a53_slot0 is a keyword or it is any general string?
Is there any convention in choosing names for cpu units?
If ‘cortex_a53_slot0’ a general string, how assembler knows it is first
integer pipeline?

How these *.md files are used? When they are compiled and how they are used?
How to verify an md file for a processor is written correctly or not? How
to test it?
What other design consideration must be kept in mind while writing a new md
file?


Thanks.

with regards,
Virendra Kumar Pathak

[Jim Wilson]

On Thu, May 7, 2015 at 4:24 AM, Virendra Kumar Pathak

Post by Virendra Kumar Pathak
I am new to gcc development. I am trying to write a new md file describing
pipeline information for a processor.
Please suggest some good reference document for understanding machine
description file.

There is some documentation for pipeline descriptions in the gcc manual.
https://gcc.gnu.org/onlinedocs/gccint/Processor-pipeline-description.html#Processor-pipeline-description

Or if you want the entire docs for the md file, you can find them here
https://gcc.gnu.org/onlinedocs/gccint/Machine-Desc.html#Machine-Desc
This is in the gcc/doc/md.texi file if you want to read the
documentation source.

Post by Virendra Kumar Pathak
For first integer pipeline following is defined - (define_cpu_unit
"cortex_a53_slot0" "cortex_a53")
Is name cortex_a53_slot0 is a keyword or it is any general string?
Is there any convention in choosing names for cpu units?
If ‘cortex_a53_slot0’ a general string, how assembler knows it is first
integer pipeline?

It is a general string, that needs to be unique. Putting the
processor name in there helps make it unique across different
processors. The rest of it is just something that makes sense for the
target, preferably something that maps to the processor documentation.

The compiler does not know that it is the first integer pipeline. All
it knows is that some following define_insn_reservation patterns use
it. These define_insn_reservation patterns will map the
cortex_a53_slot0 cpu unit to the set of instructions that can execute
on the first integer pipeline.

Post by Virendra Kumar Pathak
How these *.md files are used? When they are compiled and how they are used?

There are various genX programs that read the md file and generate a
insn-X.c file to perform actions based on info in the md file. For
instance genrecog creates the insn-recog.c file, which is for
recognizing RTL patterns that map to valid instructions on the target.
genemit creates the insn-emit.c file which emits the assembly code for
an RTL insn. Etc.

Post by Virendra Kumar Pathak
How to verify an md file for a processor is written correctly or not? How to
test it?

It is generally not possible to verify that a md file is correctly
written. All you can do is test it, and try to exercise as much of it
as possible. Testing it means compiling code, and trying various
different option combinations when compiling to exercise more of the
code. The more code you compile the better test it is. Bootstrapping
the compiler is a good correctness test. Building an entire linux
distro is a better correctness test. Running the gcc regression
testsuite is a good correctness test. You will also want to do
performance testing. Anything you can time will work. Running
standard benchmarks like SPEC is a good test. You can time how long
it takes to bootstrap gcc for instance, or maybe just pick one large
gcc source file, and time how long it takes to compile that file.

Post by Virendra Kumar Pathak
What other design consideration must be kept in mind while writing a new md
file?

If you want to contribute it to the FSF, then you will need to worry
about copyright assignments. The FSF will only accept code if they
own the copyright, and it is free of any patent claims. This can be
one of the hardest parts of contributing to GCC if you work at a
company that hasn't dealt with FSF copyright assignment paperwork
before.

Jim

[Jim Wilson]

Post by Jim Wilson
There are various genX programs that read the md file and generate a
insn-X.c file to perform actions based on info in the md file. For
instance genrecog creates the insn-recog.c file, which is for
recognizing RTL patterns that map to valid instructions on the target.
genemit creates the insn-emit.c file which emits the assembly code for
an RTL insn. Etc.

I got this wrong. genoutput and insn-output.c are for the assembly
code. genemit and insn-emit.c are for emitting the initial RTL.

Jim

[Virendra Kumar Pathak]

Hi Jim,

Thanks for the reply. This of great help.

Thanks
Virendra

Post by Jim Wilson

Post by Jim Wilson
There are various genX programs that read the md file and generate a
insn-X.c file to perform actions based on info in the md file. For
instance genrecog creates the insn-recog.c file, which is for
recognizing RTL patterns that map to valid instructions on the target.
genemit creates the insn-emit.c file which emits the assembly code for
an RTL insn. Etc.

I got this wrong. genoutput and insn-output.c are for the assembly
code. genemit and insn-emit.c are for emitting the initial RTL.
Jim