[MIPS] Backend Bugs

[witbring]

I separate the bug report #67787 since the report has distinct bugs

The subsequent issue was identified within Clang v16.0.0.

Operand Type Check Bug

Clang changes a memory operand as an immediate value in the case of bc1tl, bc1fl, jal, jalx and j instructions.

$ cat buggy3.s
    bc1tl (1)
    bc1fl (2)
    jal (3)
    jalx (4)
    j (5)

$ ./bin/clang -c --target=mips buggy3.s -o buggy3.o

$ objdump -d buggy3.o

00000000 <.text>:
   0:   45030000    bc1tl   0x4
   4:   00000000    nop
   8:   45020000    bc1fl   0xc
   c:   00000000    nop
  10:   0c000000    jal 0x0
  14:   00000000    nop
  18:   74000001    jalx    0x4
  1c:   00000000    nop
  20:   08000001    j   0x4
  24:   00000000    nop

[llvmbot]

@llvm/issue-subscribers-backend-mips

I separate the bug report #67787 since the report has distinct bugs The subsequent issue was identified within Clang v16.0.0. ### Operand Type Check Bug Clang changes a memory operand as an immediate value in the case of `bc1tl`, `bc1fl`, `jal`, `jalx` and `j` instructions. ``` $ cat buggy3.s bc1tl (1) bc1fl (2) jal (3) jalx (4) j (5) $ ./bin/clang -c --target=mips buggy3.s -o buggy3.o $ objdump -d buggy3.o 00000000 <.text>: 0: 45030000 bc1tl 0x4 4: 00000000 nop 8: 45020000 bc1fl 0xc c: 00000000 nop 10: 0c000000 jal 0x0 14: 00000000 nop 18: 74000001 jalx 0x4 1c: 00000000 nop 20: 08000001 j 0x4 24: 00000000 nop ```

[brad0]

@FlyGoat @wzssyqa Ping.

[wzssyqa]

gas complains as

nn.s: Assembler messages:
nn.s:2: Error: branch to misaligned address (0x1)
nn.s:3: Error: branch to misaligned address (0x2)
nn.s:4: Warning: no .cprestore pseudo-op used in PIC code
nn.s:6: Error: branch to misaligned address (0x5)
nn.s: Internal error (Segmentation fault).
Please report this bug.

[wzssyqa]

The real problem is that the assembler for MIPS doesn't support use IMM as the target of branch. In fact gas generates bad binary also: https://sourceware.org/bugzilla/show_bug.cgi?id=31343

[witbring]

Thank you for replay. :) I believe that an assembler ought to generate an error message whenever encountering invalid syntax.

[wzssyqa]

Yes. We have 2 choice now:

1. Add the support of IMM, as the ISA documents style;
2. Just disable this style. In fact we can use something like B . + 8.

[witbring]

Could you provide a clearer explanation? What exactly do you mean by 'Add the support of IMM'?

[FlyGoat]

@wzssyqa FYI as per my reading of MIPSpro Assembly Language Programmer’s Guide offset operand of branch instructions can only be a label (or label based expression).

However for jump intrusions it should allow arbitrary address.

http://irix7.com/techpubs/007-2418-004.pdf

[FlyGoat]

Could you provide a clearer explanation? What exactly do you mean by 'Add the support of IMM'?

I guess Yunqiang means support immediate value as memory offset properly.

[witbring]

In that case, we should take relocation into consideration.