nemanjai requested changes to this revision.
nemanjai added a comment.
This revision now requires changes to proceed.
Also if we are going to be emitting this complex sequence, in the description
of the patch, point out the test case that shows the back end handles this and
emits a single instruction.
================
Comment at: clang/lib/CodeGen/CGBuiltin.cpp:15081
}
+ case PPC::BI__builtin_ppc_cmpb: {
+ llvm::Type *Ty = Ops[0]->getType();
----------------
I find it rather surprising that we are emitting this complex sequence for this
builtin. Perhaps there is a good reason for doing so, but at the very least,
this requires a thorough explanation in a comment.
One additional concern I have with this is that if some transformation proves
that some portion of this is unused (perhaps using `DemandedBits` analysis), it
may optimize out a portion of this, thereby making the sequence emit a whole
bunch of xor's, or's, rotates, etc.
For example:
```
...
unsigned long long A = __builtin_ppc_cmpb(B, C);
return A & 0xFF00FF00FF00FF;
```
It is entirely possible that the optimizer will get rid of some of the produced
instructions and then the back end won't be able to emit a single `cmpb` but
will have to emit a whole bunch of scalar instructions.
Repository:
rG LLVM Github Monorepo
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D105194/new/
https://reviews.llvm.org/D105194
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