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optimisation can cause problems
| accessdate = 2012-04-02}}</ref> Some errors only occur under very specific circumstances and may never be detected.
| accessdate = 2012-04-02}}</ref> Some errors only occur under very specific circumstances and may never be detected.
Some compilers just perform a fast simple-minded translation from high-level source code to low-level machine language, while other compilers may optionally take (much) longer to perform the compilation by trying to improve or optimise the resulting machine code.
Some compilers just perform a fast simple-minded translation from high-level source code to low-level machine language, while other compilers may optionally take (much) longer to perform the compilation by trying to improve or optimise the resulting machine code.
==History==
==History==
Many compilers have one or more optional optimisation levels. As the level/amount of optimisation increases the compiler tends to spend more and more time doing the optimisation. The word 'optimisation' is perhaps misleading - perfect optimisation is a very hard problem ([[NP-complete]] or even [[undecidable]]), so a conceptually better term would be 'improvement'.
Many compilers have one or more optional optimisation levels. As the level/amount of optimisation increases the compiler tends to spend more and more time doing the optimisation. The word 'optimisation' is perhaps misleading - perfect optimisation is a very hard problem ([[NP-complete]] or even [[undecidable]]), so a conceptually better term would be 'improvement'.
Note that the most important criteria for judging a compiler is that it be correct, i.e. it always produces a correct translation of the high-level language, regardless of the amount of optimisation performed. The documentation for some compilers warns that results may be slightly different and/or that certain code may be incorrectly translated at high levels of optimisation.<ref>{{cite web
| url = http://www.google.co.uk/url?sa=t&rct=j&q=IBM+fortran+compiler+optimisation+level&source=web&cd=3&ved=0CEgQFjAC&url=http%3A%2F%2Fwww-01.ibm.com%2Fsupport%2Fdocview.wss%3Fuid%3Dswg27005174%26aid%3D1&ei=K-Z9T8XAKMWI8gOx76WYDg&usg=AFQjCNEGkE1JxTZVDkQvuwRK39y037UorA
| title = Code optimization with the IBM XL Compilers
| accessdate = 2012-04-05}}</ref>
Such optimisations generally make the resulting machine code 'better', for some definition of 'better':
Such optimisations generally make the resulting machine code 'better', for some definition of 'better':