How much faster if write in-line assembly rather than regular c/c++ code?
One of my senior collegues optimizes a function (he is implementing image filtering) by writing in-line assembly. Is that really necessary? Wouldn't modern compiler do that for us? Typically, how much gain do we have by converting C code into assembly?  If assembly code really brings lots of benefits, when should we convert C/C++ code into assembly and when should we leave the code as it is, since assembly code is hard to read and maintain. 

How much faster if write in-line assembly rather than regular c/c++ code?
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One of my senior collegues optimizes a function (he is implementing image filtering) by writing in-line assembly. Is that really necessary? Wouldn't modern compiler do that for us? Typically, how much gain do we have by converting C code into assembly?  If assembly code really brings lots of benefits, when should we convert C/C++ code into assembly and when should we leave the code as it is, since assembly code is hard to read and maintain. 
