Do Fortran compilers really generate faster code than C compilers

ccomparisoncompilerfortran

When I was studying in the university I often heard the idea that Fortran compilers produced faster code than C compilers for an equivalent program.

The key reasoning went like this: a Fortran compiler emits on average 1,1 processor instruction per line of code, while a C compiler emits on average 1,6 processor instruction per line of code – I don't remember the exact numbers but the idea was that C compilers emitted noticeably more machine code and therefore produced slower programs.

How valid is such comparison? Can we say that Fortran compilers produce faster programs than C compilers or vice versa and why does this difference exist?

Best Answer

IIRC one of the main reasons why Fortran is said to be faster is the absence of pointer aliasing, so they can use optimizations that C compilers can't use:

In FORTRAN, function arguments may not alias each other, and the compiler assumes they do not. This enables excellent optimization, and is one major reason for FORTRAN's reputation as a fast language. (Note that aliasing may still occur within a FORTRAN function. For instance, if A is an array and i and j are indices which happen to have the same value, then A[i] and A[j] are two different names for the same memory location. Fortunately, since the base array must have the same name, index analysis can be done to determine cases where A[i] and A[j] cannot alias.)

But I agree with others here: Comparing the average number of assembler instructions generated for a line of code is complete nonsense. For instance a modern x86 core can execute two instructions in parallel if they don't access the same registers. So you can (in theory) gain a performance increase of 100% for the same set on instructions just by reordering them. Good compilers will also often generate more assembly instructions to get faster code (think loop unrolling, inlining). The total number of assembler instructions says very little about the performance of a piece of code.

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