Neil Nelson <firstname.lastname@example.org> writes:
clang is_sorted.cpp -S -emit-llvm -o is_sorted_.ll
clang is_sorted.cpp -O0 -S -emit-llvm -o is_sorted_O0.ll
clang is_sorted.cpp -O0 -Xclang -disable-llvm-passes -S -emit-llvm -o is_sorted_disable.ll
No difference in the prior three ll files.
clang is_sorted.cpp -O1 -S -emit-llvm -o is_sorted_O1.ll
Many differences between is_sorted_O1.ll and is_sorted_.ll.
Sure. One is optimized and the other is not.
opt -O3 -S is_sorted_.ll -o is_sorted_optO3.ll
clang is_sorted.cpp -mllvm -debug-pass=Arguments -O3 -S -emit-llvm -o is_sorted_O3arg.ll
opt <optimization sequence obtained in prior step> -S is_sorted_.ll -o is_sorted_opt_parms.ll
No difference between is_sorted_optO3.ll and is_sorted_opt_parms.ll, the last two opt runs.
Ok. This isn't surprising to me.
Many differences between is_sorted_O3arg.ll and is_sorted_opt_parms.ll, the last two runs,
clang and opt.
I think the problem is that without an optimization argument (-O1, -O3,
etc.) clang sets the "optnone" attribute on all functions and opt will
refuse to optimize. I think this is very unfortunate behavior.
Given my current understanding, the ll files from the first three clang runs
are before any optimizations. Those ll files are from the front-end phase (CFE).
But this is a simple program and it may be that for a more complex program that
the ll files could be different.
Whether or not we use a -O3 optimization or use the parameters provided by clang for a
-03 optimization, we obtain the same result.
Yep, in both cases opt is not doing any optimization.
The difference in question is why an opt run using the CFE ll before optimization
obtains a different ll than a CFE run that includes optimization. That is, for this case,
it is not the expansion of the -O3 parameters that is the difference.
I think it's the optnone attribute set by clang in the first three runs.
Initially, it would be interesting to have an ll listing before optimization from the
clang run that includes optimization to compare with the ll from the clang run without
Unfortunately I don't know of a great way to do that. -mllvm
-print-before-all might be close but it will also dump out a ton of
stuff and not all dumps are complete (e.g. only dumps a Function rather
than the whole Module).