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SubjectRe: Linux

On Tue, 21 Jul 2009, Linus Torvalds wrote:

> [ Added Ian Lance Taylor to the cc, he knows the background, and unlike me
> is competent with gcc. ]
> On Tue, 21 Jul 2009, Troy Moure wrote:
>> I think I've found something interesting. Look at the the code generated
>> for edid_checksum() in driver/video/fbmon.c. This is what I see for the
>> -fno-strict-overflow kernel:
> Ooh.
> Bingo. You're 100% right, and you definitely found it (of course, there
> may be _other_ cases like this, but that's certainly _one_ of the
> problems, and probably the only one).
> Just out of curiosity, how did you find it? Now that I know where to look,
> it's very obvious in the assembler diffs, but I didn't notice it until you
> pointed it out just because there is so _much_ of the diffs...
> And yes, that's very much a compiler bug. And I also bet it's very easily
> fixed.
> The code in question is this loop:
> #define EDID_LENGTH 128
> unsigned char i, ...
> for (i = 0; i < EDID_LENGTH; i++) {
> csum += edid[i];
> all_null |= edid[i];
> }
> and gcc -fno-strict-overflow has apparently decided that that is an
> infinite loop, even though it clearly is not. So then the stupid and buggy
> compiler will compile that loop (and the whole rest of the function) to
> the "optimized" version that is just
> loop:
> jmp loop;
> I even bet I know why: it looks at "unsigned char", and sees that it is an
> 8-bit variable, and then it looks at "i < EDID_LENGTH" and sees that it is
> a _signed_ comparison (it's signed because the C type rules mean that
> 'unsigned char' will be extended to 'int' in an expression), and then it
> decides that in a signed comparison an 8-bit entry is always going to be
> smaller than 128.
> Anyway, I bet we can work around the compiler bug by just changing the
> type of "i" from "unsigned char" to be a plain "int".
> Krzysztof? Mind testing that?
> Ian? This is Linux compiled with gcc-4.2.4. I'm not seeing the
> bug in the gcc I have on my machine (gcc-4.4.0), but the bug is very clear
> (once you _find_ it, which was the problem) in the binaries that Krzysztof
> posted. They're still at:
> (Hangs)
> (OK)
> (OK)
> and you can clearly see the 'edid_checksum' miscompilation in the objdump
> disassembly.

BTW: here is a simple testcase for this bug:

--- fno-strict-overflow-fixed-bug.c ---
#include <stdio.h>

int main() {

unsigned char i;

for (i = 0; i < 128; i++)
printf("loop %u\n", i);

return 0;
--- cut here ---

The code should be compiled with:
cc -o fno-strict-overflow-fixed-bug -Os -fno-strict-overflow fno-strict-overflow-fixed-bug.c
cc -o fno-strict-overflow-fixed-bug -O2 -fno-strict-overflow fno-strict-overflow-fixed-bug.c

This bug does not exist with -O1 or if the loop is controlled by "i < 127"
or "i < 129".

So, we should make sure there is no
unsigned char i; (...) for (i = 0; i < 128; i++)
somewhere inside the kernel.

Best regards,

Krzysztof Olędzki
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