https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107066
Bug ID: 107066 Summary: Field initialized before ctor is mis-optimized away by DSE Product: gcc Version: unknown Status: UNCONFIRMED Severity: normal Priority: P3 Component: tree-optimization Assignee: unassigned at gcc dot gnu.org Reporter: fxue at os dot amperecomputing.com Target Milestone: --- By means of user-defined new operator, it is possible that a field is initialized before constructor. #include <stddef.h> class A { public: int f1; int f2; A() : f2(2) { } void *operator new(size_t size) { void *mem = ::operator new(size); A *obj = static_cast<A *>(mem); obj->f1 = 1; return obj; } }; A* foo () { return new A(); } The original gimple code of foo() is: struct A * foo () { void * D.2444; void * _9; <bb 2> : _9 = operator new (8); MEM[(struct A *)_9].f1 = 1; MEM[(struct A *)_9] ={v} {CLOBBER}; MEM[(struct A *)_9].f2 = 2; return _9; } In gimple, there exists a pseudo clobber statement marking beginning of constructor code. Although the statement is of no side effect, it is regarded as normal store by DSE when determining store redundancy. Consequently, DSE thought that "MEM[(struct A *)_9].f1 = 1" was killed by "MEM[(struct A *)_9] ={v} {CLOBBER}", and removed it. After DSE pass,the foo becomes: struct A * foo () { void * D.2444; void * _9; <bb 2> : _9 = operator new (8); MEM[(struct A *)_9] ={v} {CLOBBER}; MEM[(struct A *)_9].f2 = 2; return _9; }