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[Tinycc-devel] ARM codegen issue, possibly fixed already

From: Harald van Dijk
Subject: [Tinycc-devel] ARM codegen issue, possibly fixed already
Date: Sun, 14 Jul 2019 14:40:50 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.0


On tcc 0.9.27 and month-old git, on ARM, the following program is miscompiled:

  void f(char *a, long long b, int c) {
    *a = b < 0 && !c;

  int main(void) {
    puts("Hello, world!");
    char a;
    f(&a, 1, 0);

This fails with a segmentation fault. tcc was configured with

  ./configure --prefix=$HOME/tcc --config-arm_eabihf

and built with GCC 8.3.0. The disassembly for f shows the problem:

00000000 <f>:
   0:   e1a0c00d        mov     ip, sp
   4:   e92d000f        push    {r0, r1, r2, r3}
   8:   e92d5800        push    {fp, ip, lr}
   c:   e1a0b00d        mov     fp, sp
  10:   e24bd004        sub     sp, fp, #4
  14:   e59b000c        ldr     r0, [fp, #12]
  18:   e59b1018        ldr     r1, [fp, #24]
  1c:   e3511000        cmp     r1, #0
  20:   ca00000a        bgt     50 <f+0x50>
  24:   1a000002        bne     34 <f+0x34>
  28:   e59b1014        ldr     r1, [fp, #20]
  2c:   e3511000        cmp     r1, #0
  30:   2a000006        bcs     50 <f+0x50>
  34:   e50b0004        str     r0, [fp, #-4]
  38:   e59b001c        ldr     r0, [fp, #28]
  3c:   e3500000        cmp     r0, #0
  40:   0a000000        beq     48 <f+0x48>
  44:   ea000001        b       50 <f+0x50>
  48:   e3a00001        mov     r0, #1
  4c:   ea000000        b       54 <f+0x54>
  50:   e3a00000        mov     r0, #0
  54:   e51b1004        ldr     r1, [fp, #-4]
  58:   e5c10000        strb    r0, [r1]
  5c:   e89ba800        ldm     fp, {fp, sp, pc}

At <14>, the value of <a> is read. At <34>, that value is saved on the stack. At <54>, that value is loaded again, and at <58>, the memory referenced by <a> is written to.

The problem with that is that the branches at <20> and <30> go straight to <50>, skipping the saving of the value, and whatever value happened to be there on the stack is used instead.

This particular program no longer crashes since recently: the crash no longer happens after

commit 8227db3a23fd3cf11840eaa25eab5f3f5f813ac7
Author: grischka <grischka>
Date:   Sat Jun 22 11:45:35 2019 +0200

    jump optimizations

    This unifies VT_CMP with VT_JMP(i) by using mostly VT_CMP
    with both a positive and a negative jump target list.

    Such we can delay putting the non-inverted or inverted jump
    until we can see which one is nore suitable (in most cases).

        if (a && b || c && d)
            e = 0;

As that commit is an optimization, not a codegen fix, I would like to ask whether the original issue is still present and just requires a more complex test case to expose, or whether it has accidentally been fixed by this commit.

Thanks and keep up the good work!

Harald van Dijk

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