Hi,
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).
example:
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!
Cheers,
Harald van Dijk
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