I am interested to know how to get the most optimised IR (the compiler won't do any higher level opt but only translates it down to MC).
I tried to emit LLVM IR in clang by using '-S -O3 -emit-llvm'. Then I tried to use 'opt' to optimise it, but it seems to produce the same code.
Can I assume that LLVM will not do any code transformation on this level of IR but directly translate this IR into MC?
Is there any internal lower-level IR that LLVM uses during the machine code generation?
Can I assume that LLVM will not do any code transformation on this level of
IR but directly translate this IR into MC?
There are a few passes that happen on IR even after this phase, but
not many. The idea is that they're the passes that are more
target-specific, though that's probably less true now than it was
before. Giving "-print-after-all" to llc currently lists at least some
of them.
(I suspect it's skipping Loop strength reduction for some reason, but
that definitely runs from llc).
Is there any internal lower-level IR that LLVM uses during the machine code
generation?
Yes, after those few IR passes, the code is converted to
MachineInstrs. The actual instructions are target-specific, and most
of them represent CPU instructions in 1:1 correspondence, but there
are a few pseudo-instructions there that may expand to more (or less).
Lots more passes run on that representation (register allocation and
scheduling being the most obvious). "-print-after-all" shows them too,
and the IR-equivalent at each stage.
Finally, the MachineInstrs get lowered to MCInsts and then printed and
discarded almost immediately (to the extent that MCInsts tend to only
exist on the stack; they don't stick around long enough to need heap
allocation).