CLang Optimizations.md

CLang optimizations on Mac OSX

Version:

Apple LLVM version 6.0 (clang-600.0.57) (based on LLVM 3.5svn)
Target: x86_64-apple-darwin14.1.0
Thread model: posix

This was made with commands:

echo 'int;' | clang -xc -O0    - -o /dev/null -\#\#\#
echo 'int;' | clang -xc -O1    - -o /dev/null -\#\#\#
echo 'int;' | clang -xc -O2    - -o /dev/null -\#\#\#
echo 'int;' | clang -xc -O3    - -o /dev/null -\#\#\#
echo 'int;' | clang -xc -Ofast - -o /dev/null -\#\#\#
echo 'int;' | clang -xc -Os    - -o /dev/null -\#\#\#
echo 'int;' | clang -xc -Oz    - -o /dev/null -\#\#\#

• -O0 means "no optimization": this level compiles the fastest and generates the most debuggable code. It enable -mrelax-all option.
• -O1 somewhere between -O0 and -O2.
• -O2 moderate level of optimization which enables most optimizations.
• -O3 is like -O2 except that it enables optimizations that take longer to perform or that may generate larger code (in an attempt to make the program run faster).
• -Ofast enable -O3, with other aggressive optimizations that may violate strict compliance with language standards. It speedup math calculations.
• -Os is like -O2 with extra optimizations to reduce code size.
• -Oz is like -Os, but try to minimise even more the code size.

Option	-O0	-O1	-O2	-O3	-Ofast	-Os	-Oz	Description
-triple x86_64-apple-macosx10.10.0	✅	✅	✅	✅	✅	✅	✅	Specify target triple
-emit-obj	✅	✅	✅	✅	✅	✅	✅	Emit native object files
-mrelax-all	✅	❌	❌	❌	❌	❌	❌	(integrated-as) Relax all machine instructions
-disable-free	✅	✅	✅	✅	✅	✅	✅	Disable freeing of memory on exit
-disable-llvm-verifier	✅	✅	✅	✅	✅	✅	✅	Don't run the LLVM IR verifier pass
-mrelocation-model pic	✅	✅	✅	✅	✅	✅	✅	The relocation model to use
-pic-level 2	✅	✅	✅	✅	✅	✅	✅	Value for __PIC__
-mdisable-fp-elim	✅	✅	✅	✅	✅	✅	✅	Disable frame pointer elimination optimization
-menable-no-infs	❌	❌	❌	❌	✅	❌	❌	Allow optimization to assume there are no infinities.
-menable-no-nans	❌	❌	❌	❌	✅	❌	❌	Allow optimization to assume there are no NaNs.
-menable-unsafe-fp-math	❌	❌	❌	❌	✅	❌	❌	Allow unsafe floating-point math optimizations which may decrease precision
-ffp-contract=fast	❌	❌	❌	❌	✅	❌	❌	Form fused FP ops (e.g. FMAs): fast (everywhere) OR on (according to FP_CONTRACT pragma, default) OR off (never fuse)
-ffast-math	❌	❌	❌	❌	✅	❌	❌	Enable the frontend's 'fast-math' mode. This has no effect on optimizations, but provides a preprocessor macro __FAST_MATH__ the same as GCC's -ffast-math flag
-masm-verbose	✅	✅	✅	✅	✅	✅	✅	Generate verbose assembly output
-munwind-tables	✅	✅	✅	✅	✅	✅	✅	Generate unwinding tables for all functions
-target-cpu core2	✅	✅	✅	✅	✅	✅	✅	Target a specific cpu type
-target-linker-version 241.9	✅	✅	✅	✅	✅	✅	✅	Target linker version
-resource-dir	✅	✅	✅	✅	✅	✅	✅	The directory which holds the compiler resource files
-fdebug-compilation-dir	✅	✅	✅	✅	✅	✅	✅	The compilation directory to embed in the debug info.
-ferror-limit 19	✅	✅	✅	✅	✅	✅	✅	Set the maximum number of errors to emit before stopping (0 = no limit).
-fmessage-length 117	✅	✅	✅	✅	✅	✅	✅	Format message diagnostics so that they fit within N columns or fewer, when possible.
-stack-protector 1	✅	✅	✅	✅	✅	✅	✅	Enable stack protectors
-mstackrealign	✅	✅	✅	✅	✅	✅	✅	Force realign the stack at entry to every function
-fblocks	✅	✅	✅	✅	✅	✅	✅	Enable the 'blocks' language feature
-fobjc-runtime	✅	✅	✅	✅	✅	✅	✅	Specify the target Objective-C runtime kind and version
-fencode-extended-block-signature	✅	✅	✅	✅	✅	✅	✅	enable extended encoding of block type signature
-fdiagnostics-show-option	✅	✅	✅	✅	✅	✅	✅	Print option name with mappable diagnostics
-fcolor-diagnostics	✅	✅	✅	✅	✅	✅	✅	Use colors in diagnostics
-vectorize-loops	❌	❌	✅	✅	✅	✅	❌	Run the Loop vectorization passes
-vectorize-slp	✅	✅	✅	✅	✅	✅	✅	Run the SLP vectorization passes

Thanks for this, super helpful!