I have the following code. It is compiled at -O3 using g++
with the -std=gnu++14 option.
The code in question is the following:
if ( !testbit(rec_scan0,63) && f20type == URI ) {
Valgrind claims that the "Conditional jump or move depends on uninitialized value(s)"
Well, only if !testbit(rec_scan0,63) should we check the f20type value,
right? Apparently GCC is reordering the expression. This makes no
sense to me, from the old school of C coding.
My question is 2 fold:
1 - is this legal (and I think it is) and if so would someone point to the relevant
part of the C++ standard. (I can’t find it)
2 - Is there a flag to g++ to disable this optimization only? I don’t see one that
is directly related to this kind of thing.
The generated assembly looks like the following:
346d: 89 c7 mov %eax,%edi
346f: 88 84 24 a2 00 00 00 mov %al,0xa2(%rsp)
3476: 83 f7 01 xor $0x1,%edi // set (or clear) low bit
3479: 83 7c 24 20 16 cmpl $0x16,0x20(%rsp) // check for == URI
347e: 75 09 jne 3489 <_run+0x2b89> // jump != URI
3480: 40 84 ff test %dil,%dil // check testbit result
3483: 0f 85 77 17 00 00 jne 4c00 <_run+0x4300> // jump based on testbit result
David Barto
barto@xxxxxxxxxxxxxxxxxxxxxx
Sometimes, my best code does nothing. Most of the rest of it has bugs.
