[PowerPC] Handle cmp op promotion for SELECT[_CC] nodes in PPCTL::DAGCombineExtBoolTrunc

PPCTargetLowering::DAGCombineExtBoolTrunc contains logic to remove unwanted
truncations and extensions when dealing with nodes of the form:
  zext(binary-ops(binary-ops(trunc(x), trunc(y)), ...)

There was a FIXME in the implementation (now removed) regarding the fact that
the function would abort the transformations if any of the non-output operands
of a SELECT or SELECT_CC node would need to be promoted (because they were
also output operands, for example). As a result, we continued to generate
unnecessary zero-extends for code such as this:

  unsigned foo(unsigned a, unsigned b) {
    return  (a <= b) ? a : b;
  }

which would produce:

  cmplw 0, 3, 4
  isel 3, 4, 3, 1
  rldicl 3, 3, 0, 32
  blr

and now we produce:

  cmplw 0, 3, 4
  isel 3, 4, 3, 1
  blr

which is better in the obvious way.

llvm-svn: 224213

1 files changed, 11 insertions, 0 deletions

diff --git a/llvm/test/CodeGen/PowerPC/rm-zext.ll b/llvm/test/CodeGen/PowerPC/rm-zext.ll
index 1e60d1198bc..8fab38ad4cf 100644
--- a/llvm/test/CodeGen/PowerPC/rm-zext.ll
+++ b/llvm/test/CodeGen/PowerPC/rm-zext.ll
@@ -28,5 +28,16 @@ entry:
 ; CHECK: blr
 }
 
+define zeroext i32 @min(i32 zeroext %a, i32 zeroext %b) #0 {
+entry:
+  %cmp = icmp ule i32 %a, %b
+  %cond = select i1 %cmp, i32 %a, i32 %b
+  ret i32 %cond
+
+; CHECK-LABEL @min
+; CHECK-NOT: rldicl 3, {{[0-9]+}}, 0, 32
+; CHECK: blr
+}
+
 attributes #0 = { nounwind readnone }