https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65449
--- Comment #4 from ma.jiang at zte dot com.cn --- Hi Bernd, Thanks for your apply. (In reply to Bernd Edlinger from comment #3) > Yes, but that is not the fault of the strict volatile code path any more. // Sure,I agree with you. The redundant read is another problem. In fact, it should be named as "volatile pointer dereference produce redundant read". > For bit-fields this redundant read is exactly what AAPCS demands: > // Yes.I know that volatile bitfiled need the redundant read. That is why I thought there were connections between the redundant read and -fstrict-volatile-bitfields originally. > IMO, the problem is this: > > In store_fixed_bit_field_1 we do not know if the access is on a > packed structure member where the extra read is not necessary, > or if we have a bit-field where the extra read would be mandatory, > even if the complete container is overwritten. // I agree that the problem is caused by the store_fixed_bit_field_1.But,I disgree with you about three things.First,the redundant read should not appear if -fstrict-volatile-bitfields was off. Second, in store_fixed_bit_field_1 we can distinguish pointer dereference from structure member access(e.g.,MEM_EXPR(op0) will tell us whether op0 is a componet_ref or not, if MEM_P(op0) is true). Third, as gcc manual said "-fstrict-volatile-bitfields :This option should be used if accesses to volatile bit-fields (or other structure fields, although the compiler usually honors those types anyway) should use a single access of the width of the field’s type, aligned to a natural alignment if possible.", store_fixed_bit_field_1 does not need to distinguish bitfields from normal structure member. > > BTW: What happens in your example if you use -O0? Isn't there still an > unaligned stw access? That's because you example is in a way invalid C. > You can't set an int* to an unaligned address, it must be at least > aligned to the GET_MODE_ALIGNMENT(SImode). //Yes, I know what you mean. Split access is a auxiliary fuction provided by compiler with the help of data analysis.As -O0 turn off all analysis , an unaligned stw is acceptable. And ,BTW, the C standard said nothing about unaligned access as I know. Could you show me something about it?
