Use PPC reciprocal estimates with Newton iteration in fast-math mode

When unsafe FP math operations are enabled, we can use the fre[s] and
frsqrte[s] instructions, which generate reciprocal (sqrt) estimates, together
with some Newton iteration, in order to quickly generate floating-point
division and sqrt results. All of these instructions are separately optional,
and so each has its own feature flag (except for the Altivec instructions,
which are covered under the existing Altivec flag). Doing this is not only
faster than using the IEEE-compliant fdiv/fsqrt instructions, but allows these
computations to be pipelined with other computations in order to hide their
overall latency.

I've also added a couple of missing fnmsub patterns which turned out to be
missing (but are necessary for good code generation of the Newton iterations).
Altivec needs a similar fix, but that will probably be more complicated because
fneg is expanded for Altivec's v4f32.

llvm-svn: 178617

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCISelLowering.h b/llvm/lib/Target/PowerPC/PPCISelLowering.h
index 6690899e5d2..2ddcb25a23e 100644
--- a/llvm/lib/Target/PowerPC/PPCISelLowering.h
+++ b/llvm/lib/Target/PowerPC/PPCISelLowering.h
@@ -49,6 +49,9 @@ namespace llvm {
       /// unsigned integers.
       FCTIDUZ, FCTIWUZ,
 
+      /// Reciprocal estimate instructions (unary FP ops).
+      FRE, FRSQRTE,
+
       // VMADDFP, VNMSUBFP - The VMADDFP and VNMSUBFP instructions, taking
       // three v4f32 operands and producing a v4f32 result.
       VMADDFP, VNMSUBFP,
@@ -620,6 +623,10 @@ namespace llvm {
 
     SDValue lowerEH_SJLJ_SETJMP(SDValue Op, SelectionDAG &DAG) const;
     SDValue lowerEH_SJLJ_LONGJMP(SDValue Op, SelectionDAG &DAG) const;
+
+    SDValue DAGCombineFastRecip(SDNode *N, DAGCombinerInfo &DCI,
+                                bool UseOperand = true) const;
+    SDValue DAGCombineFastRecipFSQRT(SDNode *N, DAGCombinerInfo &DCI) const;
   };
 }