[llvm-dev] Difference between clang -O1 -Xclang -disable-O0-optnone and clang -O0 -Xclang -disable-O0-optnone in LLVM 9

[hameeza ahmed via llvm-dev]

Hello,
I m trying to test individual O3 optimizations/ replicating O3 behavior on
IR. I took unoptimized IR (O0), used disable-o0-optnone via (*clang -O0
 -Xclang -disable-O0-optnone*). I read somewhere about *clang -O1  -Xclang
-disable-O0-optnone,* so I also tested on this initial IR.

I have observed by using individual optimizations, the performance (i.e
time) is better when the base/initial IR is generated via  *clang -O1
 -Xclang -disable-O0-optnone*. In case of *clang -O0  -Xclang
-disable-O0-optnone* initial IR, the performance is reduced.

What is the possible reason for this?

What is the right way?
Please guide.
Thank You
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