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> “OK, set your optimizer to -O0,”, I told Jay, “and test. If it fails to segfault, you have an optimizer bug. Walk the optimization level upwards until the bug

> “OK, set your optimizer to -O0,”, I told Jay, “and test. If it fails to segfault, you have an optimizer bug. Walk the optimization level upwards until the bug reproduces, then back off one.” That is 99.9% rubbish. By which I mean in 99.9% of cases, when code works at -O0 and not at higher optimization levels your code has a bug, almost certainly involving invoking undefined behaviour. Certainly it is possible to find optimiser bugs, I have done it myself, but it is many times more common for your code to have bugs. Now, you could argue that optimizers shouldn't make such extensive use of undefined behaviour, which leads to these kinds of bugs, but it is perfectly valid for a C compiler to do so.

cpncrunch 13y ago
Agreed. Many times I've seen a bug disappear when I've added printfs or turned on -g, but I've never, ever seen an optimization bug in a C/C++ compiler. The only time I've seen optimization bugs is in the emscripten compiler, but I'm sure those will be ironed out eventually (and the current version seems pretty stable).
- infogulch 13y ago
  That has happened to me as well. There's a bug until I add a printf or enable debugging, or (paradoxically) go from -O0 to -O2. What's going on here? Is there some specific class of UB that I'm invoking to get this infuriating bug?
  - cpncrunch 13y ago
    I think it's usually just accessing out of bounds memory / uninitialized read, and -g/-O just happens to change the memory layout enough to cause/prevent the crash.
- userbinator 13y ago
  In nearly 2 decades I have seen only one compiler bug, in MSVC6, which was related to large switch statements. The most successful way of finding what's causing these "heisenbugs" is to NOT change the binary, but just load it in the debugger as-is, make a guess at where it's failing, and set a few breakpoints/watchpoints. Either the compiler is not generating the code that you expect, or your expectations of the language were wrong, and both cases are fairly clear from looking at the generated code.
username223 13y ago
At some point many hundreds of years ago, the law became too complicated for people to navigate on their own, so lawyers (and Law French) were created. How long will it be before there are paid language lawyers, arguing for compilers vs. programs in an ANSI/ISO "court"? There's probably money to be made in "optimizer trolling," come to think of it.

cpncrunch 13y ago

Agreed. Many times I've seen a bug disappear when I've added printfs or turned on -g, but I've never, ever seen an optimization bug in a C/C++ compiler. The only time I've seen optimization bugs is in the emscripten compiler, but I'm sure those will be ironed out eventually (and the current version seems pretty stable).

infogulch 13y ago
That has happened to me as well. There's a bug until I add a printf or enable debugging, or (paradoxically) go from -O0 to -O2. What's going on here? Is there some specific class of UB that I'm invoking to get this infuriating bug?
- cpncrunch 13y ago
  I think it's usually just accessing out of bounds memory / uninitialized read, and -g/-O just happens to change the memory layout enough to cause/prevent the crash.
userbinator 13y ago
In nearly 2 decades I have seen only one compiler bug, in MSVC6, which was related to large switch statements. The most successful way of finding what's causing these "heisenbugs" is to NOT change the binary, but just load it in the debugger as-is, make a guess at where it's failing, and set a few breakpoints/watchpoints. Either the compiler is not generating the code that you expect, or your expectations of the language were wrong, and both cases are fairly clear from looking at the generated code.

username223 13y ago

At some point many hundreds of years ago, the law became too complicated for people to navigate on their own, so lawyers (and Law French) were created. How long will it be before there are paid language lawyers, arguing for compilers vs. programs in an ANSI/ISO "court"? There's probably money to be made in "optimizer trolling," come to think of it.