riscvarchive / riscv-binutils-gdb

RISC-V backports for binutils-gdb. Development is done upstream at the FSF.

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Objdump and auipc: the immediate value should be displayed shifted #268

Closed ronan-lashermes closed 2 years ago

ronan-lashermes commented 2 years ago

Bug description

Here are some extracts from objdump desassembling (objdump -D). (Using up-to-date riscv-gnu-toolchain that points to riscv-binutils-gdb commit 116a737f438d03a1bd6aa706b6ea0b4022f3b7e2)

  48:  00003117             auipc   sp,0x3
 1dc:  00001197             auipc   gp,0x1
 1f8:  00001517             auipc   a0,0x1

Since, in auipc, the immediate encodes for bits 31 down to 12. The displayed immediate value would be easier to interpret if left shifted by 12:

  48:  00003117             auipc   sp,0x3000
 1dc:  00001197             auipc   gp,0x1000
 1f8:  00001517             auipc   a0,0x1000

aswaterman commented 2 years ago

Not a bug so much as a quirk of the RISC-V assembly language. Anyway, this change can't be made for practical reasons, because it's backwards incompatible: assembling auipc sp, 0x3000 would no longer mean the same thing as it used to.

Note that the auipc assembly syntax matches lui, which in turn matches MIPS's lui, which also keeps the argument right-justified. So, quirky as this is, it's not unique to RISC-V.

ronan-lashermes commented 2 years ago

Fair enough. Never worked with MIPS before, so that was a new "subtlety" for me with RISC-V.