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Note... it's not really that "it's never a compiler bug," but more like "it's never a backend/codegen bug." It's not particularly hard (for someone who knows t

Note... it's not really that "it's never a compiler bug," but more like "it's never a backend/codegen bug." It's not particularly hard (for someone who knows the language rules, which are difficult for a language like C++) to make a widely-used compiler be erroneous in its acceptance or rejection of code. What's much more difficult ("never" happens) is to make the compiler accept valid code and then generate an incorrect executable. It's possible (and I run into this maybe once a year doing unusual things) but it's really rare. If you think that's what's going on, it's very unlikely to be the case.

UncleEntity 2y ago
Round 'bout 10 years ago I was working on this Python C extension and, after a distro upgrade, it started segfaulting. Dropping down into gdb, python was fairly obviously calling the wrong C function. I didn't know if the linker, compiler or python was at fault and "it is never a compiler error" was at the forefront of my mind so I never even tried to report the incorrect behavior out of fear that maybe I was doing something stupid that caused gcc to compile an incorrect shared library without complaining. IIRC after the next fedora release everything started working again so maybe not me? Still don't know.
ynik 2y ago
Codegen bugs are not particularly rare either; but you usually run into them if doing "weird stuff" (which hits an edge case somewhere within the compiler). And the first instinct of most C++ programmers when seeing weird compiler behavior is to assume their weird code somehow triggered undefined behavior, so they refactor their program until it's less weird. But then it usually also no longer hits the edge case in the compiler's logic, so the program starts working correctly. Most developers then don't spend additional hours/days to investigate whether it was truly undefined behavior or if they hit a compiler bug.
titzer 2y ago
Compiler backends are really complicated and a lot more difficult to debug than frontends. In my experience they account for more than frontend bugs.
maginx 2y ago
That's not my experience - I've found a handful of accepted and verified bugs in major commercial compilers and all were in the codegen/backend, and the code to be generated was quite simple. In one case it was basically an array copy in Java byte code that got erroneously translated into what was effectively a "copy until zero termination" error.

UncleEntity 2y ago

Round 'bout 10 years ago I was working on this Python C extension and, after a distro upgrade, it started segfaulting. Dropping down into gdb, python was fairly obviously calling the wrong C function. I didn't know if the linker, compiler or python was at fault and "it is never a compiler error" was at the forefront of my mind so I never even tried to report the incorrect behavior out of fear that maybe I was doing something stupid that caused gcc to compile an incorrect shared library without complaining. IIRC after the next fedora release everything started working again so maybe not me? Still don't know.

ynik 2y ago

Codegen bugs are not particularly rare either; but you usually run into them if doing "weird stuff" (which hits an edge case somewhere within the compiler). And the first instinct of most C++ programmers when seeing weird compiler behavior is to assume their weird code somehow triggered undefined behavior, so they refactor their program until it's less weird. But then it usually also no longer hits the edge case in the compiler's logic, so the program starts working correctly. Most developers then don't spend additional hours/days to investigate whether it was truly undefined behavior or if they hit a compiler bug.

titzer 2y ago

Compiler backends are really complicated and a lot more difficult to debug than frontends. In my experience they account for more than frontend bugs.

maginx 2y ago

That's not my experience - I've found a handful of accepted and verified bugs in major commercial compilers and all were in the codegen/backend, and the code to be generated was quite simple. In one case it was basically an array copy in Java byte code that got erroneously translated into what was effectively a "copy until zero termination" error.