Backend Bugs

  1. 出现bug的源码:

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    int foo() {
      return 5;
    }

    int main() {
      putint(foo());
      return 0;
    }

  2. 目前生成的目标代码: (简化过)

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    .data
    .text
    ifElseIf:
    ifElseIfEntry:
    	li a0, 5
    	ret

    .globl main
    main:
    mainEntry:
    	# reserve space
    	addi sp, sp, -4

    	# save caller saved regs
    	addi sp, sp, -8
    	sd ra, 0(sp)

    	# call ifElseIf
    	call ifElseIf

    	# restore caller saved regs
    	ld ra, 0(sp)
    	addi sp, sp, 8

    	# get address of local var:ifElseIf
    	sw a0, 0(sp)

    	# prepare params

    	# fetch variables

    	# get address of local var:ifElseIf
    	lw t1, 0(sp)
    	mv a0, t1


    	# save caller saved regs
    	addi sp, sp, -8
    	sd ra, 0(sp)

    	call putint

    	# restore caller saved regs
    	ld ra, 0(sp)
    	addi sp, sp, 8

    	# fetch variables
    	li a0, 0

    	addi sp, sp, 4
    	ret

  3. 报错信息 (昉星光2) :

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    Bus error

  4. 修改代码使得 sp 按照8字节对齐: (即 sp 的当前值 永远与 sp初始值 相差 8*k 个字节):

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    .data
    .text
    ifElseIf:
    ifElseIfEntry:
    	li a0, 5
    	ret

    .globl main
    main:
    mainEntry:
    	# reserve space
    	addi sp, sp, -8

    	# save caller saved regs
    	addi sp, sp, -8
    	sd ra, 0(sp)

    	# call ifElseIf
    	call ifElseIf

    	# restore caller saved regs
    	ld ra, 0(sp)
    	addi sp, sp, 8

    	# get address of local var:ifElseIf
    	sw a0, 0(sp)

    	# prepare params

    	# fetch variables

    	# get address of local var:ifElseIf
    	lw t1, 0(sp)
    	mv a0, t1


    	# save caller saved regs
    	addi sp, sp, -8
    	sd ra, 0(sp)

    	call putint

    	# restore caller saved regs
    	ld ra, 0(sp)
    	addi sp, sp, 8

    	# fetch variables
    	li a0, 0

    	addi sp, sp, 8
    	ret

  5. 报错消失, 输出正确答案 5

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