How to understand the Crash triggered by Clang compiler optimization

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If someone told you that the following C++ function would cause the program crash, what reasons would you think of?

Std::string b2s (bool b) {return b? "true": "false";}

If you give some more descriptions, such as:

Crash will reappear with a certain probability.

Crash is caused by a segment error (SIGSEGV)

The Backtrace on the spot is often incomplete or even completely lost.

Only when the optimization level is above-O2 will it be (easier) to reproduce.

It can only be reproduced under Clang, but not GCC.

Well, some old birds may already have a clue, here is a minimized reproduction procedure and steps:

/ / file crash.cpp#include # include std::string _ _ attribute__ ((noinline)) b2s (bool b) {return b? "true": "false";} union {unsigned char c; bool b;} volatile utint main () {u.c = 0x80; std::cout