Assembler: Branch Instructions

[musicist288]

I'm running into a strange issue issue with the riscv assembler related to branch instructions.

I just compiled the toolchain which built the assembler from this commit: d91cadb45f3ef9f96c6ebe8ffb20472824ed05a7`

I wrote a toy program with some branch instructions and ran: riscv64-unknown-elf-as -mabi=ilp32 -march=rv32i branch-test.s -o branch-test.elf

on

branch-test.s

main:
    li      a0,1
    li      a1,-1
    li      a3,1
    li      a4,2
    bne     a0,a3,1
    beq     a0,a1,1
    bge     a1,a0,1
    bgeu    a3,a4,1
    blt     a4,a3,1
    bltu    a1,a0,1
    li      ra,5

Then I disassembled the file and got the following output:

/opt/riscv/bin/riscv64-unknown-elf-objdump -S branch-test.elf

branch-test.elf:     file format elf32-littleriscv

Disassembly of section .text:

00000000 <main>:
main():
   0:   00100513            li  a0,1
   4:   fff00593            li  a1,-1
   8:   00100693            li  a3,1
   c:   00200713            li  a4,2
  10:   00d50463            beq a0,a3,18 <main+0x18>
  14:   0000006f            j   14 <main+0x14>
  18:   00b51463            bne a0,a1,20 <main+0x20>
  1c:   0000006f            j   1c <main+0x1c>
  20:   00a5c463            blt a1,a0,28 <main+0x28>
  24:   0000006f            j   24 <main+0x24>
  28:   00e6e463            bltu    a3,a4,30 <main+0x30>
  2c:   0000006f            j   2c <main+0x2c>
  30:   00d75463            bge a4,a3,38 <main+0x38>
  34:   0000006f            j   34 <main+0x34>
  38:   00a5f463            bgeu    a1,a0,40 <main+0x40>
  3c:   0000006f            j   3c <main+0x3c>
  40:   00500093            li  ra,5

It looks to me that the branch instructions got swapped. For example, the instruction at 0x10 disassembled to beq when in the source file, it was bne. Is this an issue with the assembler/disassembler of is there something I'm doing that's causing this issue?

[aswaterman]

The linker thinks that the branch target is far away, and so instead of a simple branch, it emits a branch around a jump, which has further reach. If you make the branch target something nearby (e.g. locally declare a label and branch to that), you'll see what you expect.

On Wed, Nov 27, 2019 at 3:00 PM Joseph Rossi notifications@github.com wrote:

I'm running into a strange issue issue with the riscv assembler related to branch instructions.

I just compiled the toolchain which built the assembler from this commit: d91cadb https://github.com/riscv/riscv-binutils-gdb/commit/d91cadb45f3ef9f96c6ebe8ffb20472824ed05a7 `

I wrote a toy program with some branch instructions and ran: riscv64-unknown-elf-as -mabi=ilp32 -march=rv32i branch-test.s -o branch-test.elf

on

branch-test.s

main: li a0,1 li a1,-1 li a3,1 li a4,2 bne a0,a3,1 beq a0,a1,1 bge a1,a0,1 bgeu a3,a4,1 blt a4,a3,1 bltu a1,a0,1 li ra,5

Then I disassembled the file and got the following output:

/opt/riscv/bin/riscv64-unknown-elf-objdump -S branch-test.elf

branch-test.elf: file format elf32-littleriscv

Disassembly of section .text:

00000000

: main(): 0: 00100513 li a0,1 4: fff00593 li a1,-1 8: 00100693 li a3,1 c: 00200713 li a4,2 10: 00d50463 beq a0,a3,18 <main+0x18> 14: 0000006f j 14 <main+0x14> 18: 00b51463 bne a0,a1,20 <main+0x20> 1c: 0000006f j 1c <main+0x1c> 20: 00a5c463 blt a1,a0,28 <main+0x28> 24: 0000006f j 24 <main+0x24> 28: 00e6e463 bltu a3,a4,30 <main+0x30> 2c: 0000006f j 2c <main+0x2c> 30: 00d75463 bge a4,a3,38 <main+0x38> 34: 0000006f j 34 <main+0x34> 38: 00a5f463 bgeu a1,a0,40 <main+0x40> 3c: 0000006f j 3c <main+0x3c> 40: 00500093 li ra,5

It looks to me that the branch instructions got swapped. For example, the instruction at 0x10 disassembled to beq when in the source file, it was bne. Is this an issue with the assembler/disassembler of is there something I'm doing that's causing this issue?

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[musicist288]

Perfect! Thank you! Got the output I was expecting when making those changes.