[llvmgcc] Variable length array indexing miscompiled

[lattner]

Bugzilla Link	298
Resolution	FIXED
Resolved on	Feb 22, 2010 12:45
Version	1.0
OS	All

Extended Description

It looks like we are miscompiling indexes through C99 VLA's in some cases. File
this into the "wow, I didn't know you could do that" catagory. :)

Testcase:

---

int foo(int len, char arr[][len], int X) {
return arr[X][0];
}

The correct addressing arithmetic to generate is "arr + X*len", which we don't
do in this case.

-Chris

[lattner]

Here's another horrible testcase with the same issue:

int sub1 (int i, int j) {
typedef struct {
int c[i+2];
}c;
int x[10], y[10];

if (j == 2) {
memcpy (x, y, 10 * sizeof (int));
return sizeof (c);
} else
return sizeof (c) * 3;
}

int main() {
typedef struct {
int c[22];
}c;
if (sub1 (20, 3) != sizeof (c)*3)
abort ();
return 0;
}

[lattner]

This patch:
http://mail.cs.uiuc.edu/pipermail/llvm-commits/Week-of-Mon-20040503/014167.html

Fixes this testcase (test/Regression/CFrontend/2004-05-07-VarArrays.c):

int foo(int len, char arr[][len], int X) {
return arr[X][0];
}

The second testcase attached to this bug is actually a bug in the non-LLVM
specific portion of GCC, so it will magically be fixed when new bits are merged in.

-Chris