Carrefour will stock bread (but don't expect it to be as good as in France) and Metro also stock bread. Some Chinese bakeries also sell french stick breads but again, they don't compare to the Western equivalent.

Are there 711s there? If so they sell awesome bread

If they have the chain RT Mart (大润发) in Xi'an, it has a white bread that their bakers bake. It comes in big ass loaves for ~8 RMB. It is called 方包 and is indeed very square with a yellow label showing a sheaf of wheat.

Often, western style bread is labeled as ”salty"  to show it isn't sweet.

What kind of bread are you looking for? I think a couple of issues back Xianese talked about bread. I think all the larger supermarkets have a bread section that has something "good enough" for most bread needs. 

As a xi'an local resident, i highly recommend you try "面包新语"（mian bao xin yu）. it is very good bread  chain stores in xi'an. there have five subbranch in center and southern part of xi'an. i don't know which area close you place. but there has one subbranch nearby the bell tower. the specific address is  NO.26, fen xiang street, the south avenue, bei lin district(碑林区南大街粉巷26号)  this store very close the xi'an first hospital. (西安市第一人民医院).

go try it and you find out whether i am right or not.

walmart

7-11 is the best bread in Zhong Guo, you can get it at jinkelong too. 

Though living in Xi'an for 5 years, I find you  are  more familiar with Xi'an than me. You are really great explorers.