[llvm-bugs] [Bug 43274] New: [ARM] llvm-objdump can't disassemble multiply instructions on pre-armv6

via llvm-bugs llvm-bugs at lists.llvm.org
Wed Sep 11 04:45:37 PDT 2019


            Bug ID: 43274
           Summary: [ARM] llvm-objdump can't disassemble multiply
                    instructions on pre-armv6
           Product: libraries
           Version: 8.0
          Hardware: Other
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: ARM
          Assignee: unassignedbugs at nondot.org
          Reporter: husseydevin at gmail.com
                CC: llvm-bugs at lists.llvm.org, peter.smith at linaro.org,
                    Ties.Stuij at arm.com

Apparently, LLVM attempts to put an <unknown> if there is an illegal register
layout for the mul family of instructions on pre-ARMv6.

However, for whatever reason, instead, ALL mul instructions (even valid ones)
are disassembled as an <unknown>:

clang -O3 -c --target=arm-none-eabi -march=armv4t -fomit-frame-pointer mul.c

#include <stdint.h>

uint64_t mul(uint64_t a, uint64_t b)
    return a * b;

llvm-objdump -d mul.o

mul.o: file format ELF32-arm-little

Disassembly of section .text:
0000000000000000 mul:
       0:       10 40 2d e9     push    {r4, lr}
       4:       92 c0 8e e0  <unknown>
       8:       92 e1 24 e0  <unknown>
       c:       93 40 21 e0  <unknown>
      10:       0c 00 a0 e1     mov     r0, r12
      14:       10 40 bd e8     pop     {r4, lr}
      18:       1e ff 2f e1     bx      lr

arm-none-eabi-objdump -d mul.o

mul.o:     file format elf32-littlearm

Disassembly of section .text:

00000000 <mul>:
   0:   e92d4010        push    {r4, lr}
   4:   e08ec092        umull   ip, lr, r2, r0
   8:   e024e192        mla     r4, r2, r1, lr
   c:   e0214093        mla     r1, r3, r0, r4
  10:   e1a0000c        mov     r0, ip
  14:   e8bd4010        pop     {r4, lr}
  18:   e12fff1e        bx      lr

I can confirm that this occurs on MUL, UMULL, MLA, and UMLAL. I haven't tested
other variants. 

This does not occur if the binary targets ARMv6+. 

This was tested on Clang 8.0.1, but I presume it also affects trunk.

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