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Retrocomputing

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    Watcom had a truly fantastic optimizer by the early-to-mid 1990s that I would have put up against almost any assembly language programmer, but I wasn't using their original tools from the late 1980s, so I didn't know if it was always so awesome. Commented Sep 14, 2020 at 20:54
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    I spent quite a bit of time looking at output from Borland Turbo C 2.0 (1988) and that compiler varied wildly between smart and dumb. Some tricks (x % 8 -> and ax,7, x = !x -> neg ax; sbb ax,ax; inc ax) were ever-present, but inefficiencies (repetitive wasteful calculations of bx when doing a lot of struct accesses, making bad decisions about stack vs. si/di for frequently-used local vars...) cancelled out the benefit somewhat. Commented Sep 15, 2020 at 13:42
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    @smitelli: GCC is still like that. When targeting the Cortex-M0, even if code loads a constant into an automatic variable before a loop, and there would be a register available to hold the value throughout the loop, gcc will still sometimes not only apply constant propagation to replace the variable with a constant, and then reload the constant on every iteration of the loop, but even end up adding an extra register move on top of that. Commented Sep 15, 2020 at 15:33
  • @smitelli: The choice of whether to use stack vs SI/DI would often depend upon whether variables were declared 'register'. Suitable use of that qualifier can make a huge difference when using Turbo C (and also, incidentally, when using gcc with the -O0 flag). On some occasions, code which makes good use of the register qualifier may be more efficient when compiled with -O0 than it would at any other optimization setting, because using -O0 will prevent gcc from making a counter-productive optimization. Commented Oct 21, 2021 at 16:51
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    One thing that the optimising compiler won't do is self-modifying code so sometimes it's still worth writing some inner loops in assembly if you think you can make use of it. Commented May 1, 2023 at 11:29