EC-10.2 From C to Assembly and Back

Work through these

• Reading the assembly your compiler produced

  Every toolchain can be asked to show the assembly rather than the finished binary. Comparing a five-line function with its output is the fastest way to learn what the machine is actually asked to do.

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• A loop, a conditional and a function call in machine terms

  A loop is a comparison and a jump backwards, a conditional is a comparison and a jump forwards, and a call saves a return address before jumping. Three patterns account for most of what you will read.

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• The stack frame, laid out

  Each active call owns a region of the stack holding its return address, saved registers and local variables. Drawing the frame for a nested call is what makes recursion and stack overflow concrete rather than mysterious.

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• Pointers and arrays as addresses and offsets

  An array index becomes an address plus an offset scaled by the element size, and a pointer is that address held in a register. Seeing the arithmetic explains why the two forms compile to the same instructions.

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• What optimisation does to your source

  A compiler may remove variables, reorder work, unroll a loop or delete a whole calculation whose result is unused. The output need not resemble your source, only produce the same observable behaviour.

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• Undefined behaviour, seen from below

  When a program does something the language does not define, the compiler is entitled to assume it never happens and optimise on that basis. The resulting code often deletes the check you thought would save you.

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