llvm dev - Apr 2020 - Fwd: exact usage of HotColdSplit pass and impact on ll files

Hi there!

Recently I saw a presentation with title "*Hot Cold Splitting Optimization
Pass In LLVM*" on youtube about hotcoldsplit LLVM pass .

I want to use this built-in pass to optimize my IR but I don't know how it
can optimize my code, I didn't find any result and contacted to the
developer and he advised me to use test at LLVM for testing this pass .

I tried *opt.ll* from llvm/test/intstrumentation/BoundsChecking and :

*opt -S -hotcoldsplit opt.ll -o opt1.ll*

The result is not what I expect because I used a diffing tool and see just
some sorts of the following has been removed from the original source :





*; CHECK: mul i64 {{.*}}, 4; CHECK: sub i64 4000, %; CHECK-NEXT: icmp ult
i64 {{.*}}, 4; CHECK-NEXT: or i1; CHECK: trap*








*; CHECK: mul i64 {{.*}}, 8; CHECK: mul i64 {{.*}}, 4; CHECK: add i64;
CHECK: sub i64 16, %; CHECK-NEXT: icmp ult i64 16, %; CHECK-NEXT: icmp ult
i64 {{.*}}, 4; CHECK-NEXT: or i1; CHECK: trap*


Btw, I attached the source and output file as the result .

As in my own code, I do not use these "* ; CHECK : *" things, I see no
change on my own code after using hotcoldsplit and in fact I do expect this
built-in pass to have impact on the real code but I didn't find this .

Can someone please clarify how this hotcoldsplit pass , affect the real
code and how exactly does it working and how I can use this built-in pass
in the proper way to optimize my code?

Thanks
Azim
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