3 ms·

Is that a compiler bug?

42 points by bbatsell 17y ago

Groxx 17y ago
... or are you just happy to see me? It's an interesting write-up in support of compiler-related bugs, with a couple examples (something sorely missed in many others).
yread 17y ago
Why doesn't gcc emit warning Wsign-compare for the first example? Why does it emit it only for C++ code?
- dadkins 17y ago
  I wondered that too. My best guess is that range of signed/unsigned char both fit in signed ints, so there's no ambiguity in promoting them to ints.
patio11 17y ago
Compiler bugs are like lupus. It's not lupus, either. Relatedly, a certain outsourcing company I can't name really, really needs to stop closing every ticket with "We investigated and it should work fine, so it is probably a framework bug." Particularly when the code at issue looks like: String someLabel = null; someLabel.split(","); // throws NPE. Probably framework bug.
dadkins 17y ago
I was skeptical about his example of a compiler bug. Take the following similar example: int bar(void) { unsigned x = 1; int y = -1; return x > y; } Surprisingly, the result is always 0, at least on every version of gcc I've tried. How could this be? It turns out unsigned's are one area where the C standard really screwed up. When comparing a signed and an unsigned value, the compiler helpfully promotes the signed (-1) to unsigned. I was surprised the given example with chars instead of ints didn't give the same result. Was it really a bug, or just ambiguity in the standard?
- fhub 17y ago
  short behaves the same as char. int, long and long long all implicitly cast. Very odd. gcc v4.4.1, x86
- ori_b 17y ago
  > When comparing a signed and an unsigned value, the compiler helpfully promotes the signed (-1) to unsigned. So what do you think the compiler should do? Convert the unsigned into a signed? Disallow comparison/assignment/addition/... without an explicit cast?
- regehr 17y ago
  It was really a bug, and they fixed it: https://bugs.launchpad.net/ubuntu/hardy/+source/gcc-4.2/+bug/256797 https://bugs.launchpad.net/ubuntu/hardy/+source/gcc-4.2/+bug...

42 points by bbatsell 17y ago

Groxx 17y ago

... or are you just happy to see me? It's an interesting write-up in support of compiler-related bugs, with a couple examples (something sorely missed in many others).

yread 17y ago

Why doesn't gcc emit warning Wsign-compare for the first example? Why does it emit it only for C++ code?

dadkins 17y ago
I wondered that too. My best guess is that range of signed/unsigned char both fit in signed ints, so there's no ambiguity in promoting them to ints.

patio11 17y ago

Compiler bugs are like lupus. It's not lupus, either. Relatedly, a certain outsourcing company I can't name really, really needs to stop closing every ticket with "We investigated and it should work fine, so it is probably a framework bug." Particularly when the code at issue looks like: String someLabel = null; someLabel.split(","); // throws NPE. Probably framework bug.

dadkins 17y ago

I was skeptical about his example of a compiler bug. Take the following similar example: int bar(void) { unsigned x = 1; int y = -1; return x > y; } Surprisingly, the result is always 0, at least on every version of gcc I've tried. How could this be? It turns out unsigned's are one area where the C standard really screwed up. When comparing a signed and an unsigned value, the compiler helpfully promotes the signed (-1) to unsigned. I was surprised the given example with chars instead of ints didn't give the same result. Was it really a bug, or just ambiguity in the standard?

fhub 17y ago
short behaves the same as char. int, long and long long all implicitly cast. Very odd. gcc v4.4.1, x86
ori_b 17y ago
> When comparing a signed and an unsigned value, the compiler helpfully promotes the signed (-1) to unsigned. So what do you think the compiler should do? Convert the unsigned into a signed? Disallow comparison/assignment/addition/... without an explicit cast?
regehr 17y ago
It was really a bug, and they fixed it: https://bugs.launchpad.net/ubuntu/hardy/+source/gcc-4.2/+bug/256797 https://bugs.launchpad.net/ubuntu/hardy/+source/gcc-4.2/+bug...