Add immediate instruction getting assembled as its compressed variant by default

[Shubhodeep Roychoudhury]

I am seeing that the addi instruction in some of the directed tests (.s files) are getting assembled into the compressed variant of addi, when compiled with the support for C extension.

Is there any flag/setting that will prevent that from taking place? I would want the addi in my tests to show up as their regular rv32i variant whereas the compressed one should be assembled only when I use c.addi in my tests.

You can follow the steps given below to recreate this issue

1. Put the following in your .s file (simple.s in this case) 

    .text

   addi    x5, x5, 12

   addi    x5, x5, 12

   addi    x5, x5, 12

   addi    x5, x5, 12

   addi    x5, x5, 12

   .end

2. Next compile the .s file and disassemble the binary

riscv64-unknown-elf-as -c -march=rv64imafdc -ggdb -o simple.o simple.s

riscv64-unknown-elf-objdump simple.o  -xsCd > simple.dis

3. The disassembly seems to be having the compressed variant instead of the regular addi

Disassembly of section .text:

0000000000000000 <.L0 >:

   0: 02b1                 addi t0,t0,12

0000000000000002 <.L0 >:

   2: 02b1                 addi t0,t0,12

0000000000000004 <.L0 >:

   4: 02b1                 addi t0,t0,12

0000000000000006 <.L0 >:

   6: 02b1                 addi t0,t0,12

0000000000000008 <.L0 >:

   8: 02b1                 addi t0,t0,12

Regards,

Shubhodeep

[Andrew Waterman]

This is by design; you're using an assembler that defaults to
supporting the C extension.

On the command line, you can use -march=<something without c>, e.g.,
-march=rv32ima.

In the source code, you can use .option norvc.

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[Richard Herveille]

[Andrew Waterman]

In both cases GAS will reject the instructions.

If you want to restrict GAS to compress only certain instructions, I'd recommend writing .option norvc initially; then for each code segment you want to allow C instructions, write .option push; .option rvc; c.addi a0,1; c.slli a0, 1; .option pop.  It's extremely verbose, but it does the right thing.

On Tue, Oct 24, 2017 at 12:27 AM, Richard Herveille <richard....@roalogic.com> wrote:

If you use either option to disable defaulting to C-instructions, will c.addi still be a valid opcode?

Maybe I should rephrase. Will either of the suggested options stop defaulting to C-instructions or disable support for C-instructions all together?

 

Thanks,

Richard

 

 

 

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[Bruce Hoult]

My days of writing hairy assembly language were before I switched to *nix/gas, but I'm guessing you could make a macro that expanded "rvc addi a0,1" to emit the relevant .option directives before and after the instruction.

On Tue, Oct 24, 2017 at 11:53 AM, Andrew Waterman <and...@sifive.com> wrote:

In both cases GAS will reject the instructions.

If you want to restrict GAS to compress only certain instructions, I'd recommend writing .option norvc initially; then for each code segment you want to allow C instructions, write .option push; .option rvc; c.addi a0,1; c.slli a0, 1; .option pop.  It's extremely verbose, but it does the right thing.

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