For ARM subtargets with useNEONForSinglePrecisionFP, double count uses

of the floating point types less than 64-bits. It's somewhat of a temporary
hack but forces more accurate modeling of register pressure and results
in fewer spills.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@123811 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 16 insertions, 0 deletions

diff --git a/lib/Target/ARM/ARMISelLowering.cpp b/lib/Target/ARM/ARMISelLowering.cpp
index 8dc4a18..f4d16fc 100644
--- a/lib/Target/ARM/ARMISelLowering.cpp
+++ b/lib/Target/ARM/ARMISelLowering.cpp
@@ -700,6 +700,16 @@ ARMTargetLowering::ARMTargetLowering(TargetMachine &TM)
   benefitFromCodePlacementOpt = true;
 }
 
+// FIXME: It might make sense to define the representative register class as the
+// nearest super-register that has a non-null superset. For example, DPR_VFP2 is
+// a super-register of SPR, and DPR is a superset if DPR_VFP2. Consequently,
+// SPR's representative would be DPR_VFP2. This should work well if register
+// pressure tracking were modified such that a register use would increment the
+// pressure of the register class's representative and all of it's super
+// classes' representatives transitively. We have not implemented this because
+// of the difficulty prior to coalescing of modeling operand register classes
+// due to the common occurence of cross class copies and subregister insertions
+// and extractions.
 std::pair<const TargetRegisterClass*, uint8_t>
 ARMTargetLowering::findRepresentativeClass(EVT VT) const{
   const TargetRegisterClass *RRC = 0;
@@ -713,6 +723,12 @@ ARMTargetLowering::findRepresentativeClass(EVT VT) const{
   case MVT::f32: case MVT::f64: case MVT::v8i8: case MVT::v4i16:
   case MVT::v2i32: case MVT::v1i64: case MVT::v2f32:
     RRC = ARM::DPRRegisterClass;
+    // When NEON is used for SP, only half of the register file is available
+    // because operations that define both SP and DP results will be constrained
+    // to the VFP2 class (D0-D15). We currently model this constraint prior to
+    // coalescing by double-counting the SP regs. See the FIXME above.
+    if (Subtarget->useNEONForSinglePrecisionFP())
+      Cost = 2;
     break;
   case MVT::v16i8: case MVT::v8i16: case MVT::v4i32: case MVT::v2i64:
   case MVT::v4f32: case MVT::v2f64: