The nightmare of every programmer are silent compiler bugs. But fortunately I didn’t have to deal with one of those. One of the runner ups for me has to be compiler errors.
The occasion for this post is me recently encountering an internal compiler error in Visual C++ 2013RC. It’s documented here and has apparently been fixed in the full version but I currently don’t have access to it and so I’m trying to work around it without updating my compiler.
The error appears somewhere in the function
static ALWAYS_INLINE void diff(BigInt& c, const BigInt& aRef, const BigInt& bRef)
which is defined in the dtoa.cpp (double to ascii apparently) that as far as I can tell originates from the WebKit project and is used for JavaScript.
Anyway, the error suggests that I simplify the code around the area where the error occurs. So, I did.
After commenting out code until the compilation works and partitioning the function to get to the exact problem area, I found out that it was
do {
unsigned long long y = (unsigned long long)*xa++ - *xb++ - borrow;
borrow = y >> 32 & (uint32_t) 1;
*xc++ = (uint32_t) y & 0xffffffffUL;
} while (xb < xbe);
while (xa < xae) {
unsigned long long y = *xa++ - borrow;
borrow = y >> 32 & (uint32_t) 1;
*xc++ = (uint32_t) y & 0xffffffffUL;
}
Which I “simplified” to this:
do {
unsigned long long y;
if (xb < xbe)
y= (unsigned long long)*xa++ - *xb++ - borrow;
else
y = *xa++ - borrow;
borrow = y >> 32 & (uint32_t) 1;
*xc++ = (uint32_t) y & 0xffffffffUL;
} while (xb < xbe || xa < xae);
I’m keenly aware of the performance characteristics of this change but I just wanted to make it compile with my current setup and so this did it. I don’t plan on using much JavaScript with QT anyway, so that shouldn’t be an issue for me anyway. Just wanted to archive this for my purposes and to help a few people who are possibly in the same situation. I might post a patch a little later.