[ruby-core:102690] [Ruby master Bug#17540] A segfault due to Clang/LLVM optimization on 32-bit ARM Linux

Issue #17540 has been updated by xtkoba (Tee KOBAYASHI).


Compilation with `-fno-strict-aliasing` seems to resolve the main issue of the `ruby` crash. It would be nice if this compiler option is added by default, unless it causes a serious performance drawback.

user:shyouhei That change might have induced the issue, but I don't think it is the fundamental cause. The trouble would be that the type `struct RBasic` has aliases. In this point of view, yet another workaround would be as follows (although I don't know whether the attribute `may_alias` works with C++):
```
--- a/include/ruby/internal/core/rbasic.h
+++ b/include/ruby/internal/core/rbasic.h
@@ -66,7 +66,7 @@ RBasic {
     {
     }
 #endif
-};
+} __attribute__ ((__may_alias__));
 
 RBIMPL_SYMBOL_EXPORT_BEGIN()
 VALUE rb_obj_hide(VALUE obj);
```

FYI, some searching told me that CPython had a similar issue until version 2.7 with its object representation in C (cf. https://bugs.python.org/issue30104).

----------------------------------------
Bug #17540: A segfault due to Clang/LLVM optimization on 32-bit ARM Linux 
https://bugs.ruby-lang.org/issues/17540#change-90685

* Author: xtkoba (Tee KOBAYASHI)
* Status: Open
* Priority: Normal
* ruby -v: ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [armv7a-linux-eabi]
* Backport: 2.5: UNKNOWN, 2.6: UNKNOWN, 2.7: UNKNOWN, 3.0: UNKNOWN
----------------------------------------
When built with `optflags=-O3` (which is the default), `ruby -e "pp Thread.main"` causes a segfault, which seems to be worked around by the following change:
```
--- a/include/ruby/internal/fl_type.h
+++ b/include/ruby/internal/fl_type.h
@@ -231,7 +231,7 @@
 RBIMPL_ATTR_PURE_UNLESS_DEBUG()
 RBIMPL_ATTR_ARTIFICIAL()
 static inline VALUE
-RB_FL_TEST_RAW(VALUE obj, VALUE flags)
+RB_FL_TEST_RAW(volatile VALUE obj, VALUE flags)
 {
     RBIMPL_ASSERT_OR_ASSUME(RB_FL_ABLE(obj));
     return RBASIC(obj)->flags & flags;
```

There might be a bug in the optimizer of Clang/LLVM (version 11.0.1).



-- 
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