(abs(i32 X, i1 1) always produces a positive result. The 'i1 1'
means INT_MIN input produces poison. If the result is sign extended,
InstCombine will convert it to zext. This does not produce ideal
code for RISCV.
This patch reverses the zext back to sext which can be folded
into a subw or negw. Ideally we'd do this in SelectionDAG, but
we lose the INT_MIN poison flag when llvm.abs becomes ISD::ABS.
Reviewed By: reames
Differential Revision: https://reviews.llvm.org/D130412
If X is known positive by a dominating condition, we can fill in
ones into the upper bits of C1 if that would allow it to become an
simm12 allowing the use of ANDI.
This pattern often occurs in unrolled loops where the induction
variable has been widened.
To get the best benefit from this, I had to move the pass above
ConstantHoisting which is in addIRPasses. Otherwise the AND constant
is often hoisted away from the AND.
Reviewed By: asb
Differential Revision: https://reviews.llvm.org/D129888
We're creating single instruction to replace another instruction.
We can insert using the InsertBefore operand of the constructor.
Then copy the debug location.
Initial optimization is to convert (i64 (zext (i32 X))) to
(i64 (sext (i32 X))) if the dominating condition for the basic block
guaranteed the sign bit of X is zero.
This frequently occurs in loop preheaders where a signed induction
variable that can never be negative has been widened. There will be
a dominating check that the 32-bit trip count isn't negative or zero.
The check here is not restricted to that specific case though.
A i32->i64 sext is cheaper than zext on RV64 without the Zba
extension. Later optimizations can often remove the sext from the
preheader basic block because the dominating block also needs a sext to
evaluate the greater than 0 check.
Reviewed By: asb
Differential Revision: https://reviews.llvm.org/D129732
