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Pusnow opened this issue Jun 12, 2020 · 5 comments
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[Homework 7] Floating point calling convention. #209

Pusnow opened this issue Jun 12, 2020 · 5 comments
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homework - asmgen src/asmgen/mod.rs

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@Pusnow
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Pusnow commented Jun 12, 2020

GotBolt maps floating point arguments to integer argument registers (a0-a7).

However, there are separate floating point registers (fa0-fa7).

RISC-V ABI also uses them:

If argument i < 8 is a floating-point type, it is passed in floating-point register fai; otherwise, it is passed in integer register ai.

So, which one should we use?
@cmpark0126 cmpark0126 self-assigned this Jun 12, 2020
@jeehoonkang jeehoonkang mentioned this issue Jun 13, 2020
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@jeehoonkang
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(continuing the discussion of #211 2.)

There are 2 ways to call a function: (1) allocating stack & (2) passing through a0, a1, a2, ... (if argument <= 8). And we know that if the function has Call Instruction, we must use the (1).
To satisfy hw7 (generate correct RISC-V code), is it possible to somehow "generalize" to only use only the (1)?

@cmpark0126
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cmpark0126 commented Jun 14, 2020 •
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Please refer to this link: https://godbolt.org/z/mkZvYQ

Godbolt provides two compilers using RISC-V: RISC-V gcc 8.2.0 and RISC-V rv64gc clang.

As can be seen in the above link, RISC-V gcc 8.2.0 follows the calling convention of the RISC-V ABI, while RISC-V rv64gc clang not to follow.

So, please implement asmgen by referring to the behavior of RISC-V gcc 8.2.0.

FYI, for the final project, riscv64-linux-gnu-gcc-10 will be used to link your solution and test driver.
(riscv64-linux-gnu-gcc-10 shows similar behavior to RISC-V gcc 8.2.0)

@cmpark0126 cmpark0126 assigned jeehoonkang and unassigned cmpark0126 Jun 15, 2020
@jeehoonkang
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jeehoonkang commented Jun 15, 2020 •
edited by minseongg
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RISC-V has multiple calling conventions regarding floating-point numbers: https://github.com/riscv-non-isa/riscv-elf-psabi-doc/blob/44f98e0fd8104def00f2a5a8d94b23dd647d18fb/riscv-elf.md#named-abis Conventions differ on the use of floating-point registers. In particular, the LP64 ABI doesn't use fa? registers, while LP64D does.

It seems clang uses LP64 by default, which I think is bug because LP64D is the default ABI for RV64GC (on which KECC is targeting). If you add -mabi=lp64d to the compiler flag, then you'll get the expected result (i.e., using fa? registers for arguments and returns).

The specification for the assignments was confusing, so I'd like to declare:

  • For homework 7, you don't need to observe the calling convention. All test functions have the signature () -> (), for which calling conventions don't matter at all.

  • For project,you need to observe the LP64D calling convention. Benchmark will ask whether you correctly implemented the calling convention.

This was referenced Jun 15, 2020
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@jeehoonkang jeehoonkang removed their assignment Jun 15, 2020
@Pusnow
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Pusnow commented Jun 19, 2020

Great! I will keep open this issue for other students.

@jeehoonkang
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I believe this issue is now sufficiently advertised :) Thank you all for making such a wonderful discussion thread.

@kimjungwow kimjungwow mentioned this issue Jul 5, 2020
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@minseongg minseongg mentioned this issue Jun 2, 2022
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@jirheee jirheee added the homework - asmgen src/asmgen/mod.rs label Dec 25, 2022
@jirheee jirheee mentioned this issue Mar 10, 2023
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@Lee-Janggun Lee-Janggun mentioned this issue Feb 27, 2025
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