4 ms·

You might be misremembering but strength reduction is part of ‘-O1’ [0]. However, my point for a general purpose language system like OpenJDK that favors multi

by oso2k 7y ago

You might be misremembering but strength reduction is part of ‘-O1’ [0]. However, my point for a general purpose language system like OpenJDK that favors multithreading or multiprocessing or server workloads, each core/thread and process/thread share the I-Cache/L1 cache. Address lines are still 64 bytes. Code for these systems rarely is expected to run in isolation. I tend to want to be a good neighbor and reduce code size when I can. [0] https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

nwallin 7y ago
I'm actually super way too drunk to give you a full rebuttal, so here's a godbolt compilation thing that proves that I'm right and you're wrong: https://cpp.godbolt.org/z/-zeMWT https://cpp.godbolt.org/z/-zeMWT (stop reading now if that's enough) The -Os compilation is some bookkeeping and an idiv instruction, the -O2 compilation is bookkeeping, two shifts, and an and instruction. I don't know what they because I'm drunk sorry not sorry but the one that does the idiv instruction is both slower and smaller and that's on purpose. (you can tell the idiv one is smaller by clicking the 11010 button to display offsets. the starting instructions of both functions have the same offset, but the final instruction of the -Os compilation is substantially smaller than the -O2 compilation.) The description of -Os which you linked is telling. It says it's -O2 without certain optimizations, and then it also says: > It also enables -finline-functions, causes the compiler to tune for code size rather than execution speed, and performs further optimizations designed to reduce code size. Somewhere buried in that tuning for code size rather than execution speed and further optimizations designed to reduce code size is an optimization that will replace bitwise magic with division statements. -Os does what it says on the tin. It makes your code small. It makes your code slow. It does so on purpose. People think that -Os is a superset of -O1 and a subset -O2. It's neither. It is neither a superset of -O1 nor -O2, nor is it a subset of -O1 or -O2. There are speed optimizations that -Os adds to -O1 and there are size optimizations that -Os adds to both -O1 and -O2. The point, I think, of -Os, is for embedded. If you have a size n PROM, and your code compiles to size n+1 with -O2, and if you apply -Os and it compiles to size n, that's a feature. -Os, in my opinion, ought to be uncompromising towards that goal. For better or for worse. I'll be sober in the morning and can engage with you better then. Sorry.
- hak8or 7y ago
  Just wanted to say, you were surely drunk when writing that, but it was still very fun to read.

by oso2k 7y ago

nwallin 7y ago

I'm actually super way too drunk to give you a full rebuttal, so here's a godbolt compilation thing that proves that I'm right and you're wrong: https://cpp.godbolt.org/z/-zeMWT https://cpp.godbolt.org/z/-zeMWT (stop reading now if that's enough) The -Os compilation is some bookkeeping and an idiv instruction, the -O2 compilation is bookkeeping, two shifts, and an and instruction. I don't know what they because I'm drunk sorry not sorry but the one that does the idiv instruction is both slower and smaller and that's on purpose. (you can tell the idiv one is smaller by clicking the 11010 button to display offsets. the starting instructions of both functions have the same offset, but the final instruction of the -Os compilation is substantially smaller than the -O2 compilation.) The description of -Os which you linked is telling. It says it's -O2 without certain optimizations, and then it also says: > It also enables -finline-functions, causes the compiler to tune for code size rather than execution speed, and performs further optimizations designed to reduce code size. Somewhere buried in that tuning for code size rather than execution speed and further optimizations designed to reduce code size is an optimization that will replace bitwise magic with division statements. -Os does what it says on the tin. It makes your code small. It makes your code slow. It does so on purpose. People think that -Os is a superset of -O1 and a subset -O2. It's neither. It is neither a superset of -O1 nor -O2, nor is it a subset of -O1 or -O2. There are speed optimizations that -Os adds to -O1 and there are size optimizations that -Os adds to both -O1 and -O2. The point, I think, of -Os, is for embedded. If you have a size n PROM, and your code compiles to size n+1 with -O2, and if you apply -Os and it compiles to size n, that's a feature. -Os, in my opinion, ought to be uncompromising towards that goal. For better or for worse. I'll be sober in the morning and can engage with you better then. Sorry.

hak8or 7y ago
Just wanted to say, you were surely drunk when writing that, but it was still very fun to read.