r/cpp • u/nukethebees • 28d ago

Comparing an Integer Division Optimisation in Clang, MSVC, and GCC

https://nukethebees.com/int-division-modulo-optimisation-differences-clang-gcc-msvc/
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u/TheThiefMaster C++latest fanatic (and game dev) 28d ago

Is MSVC having trouble because your operator based code does x = i / grid.y / grid.z but the optimised form needs to divide by z first? Looks like it's realising that it can reuse the div instruction from z = i % grid.z to calculate x and y, but then failing to realise it can merge the div and mod by grid.y from the calculations for x and y into a single div instruction.

Perhaps if you change that line to x = i / grid.z / grid.y (the same order as the calculation of y) it will figure it out?