[PATCH] rs6000: Improve comparison rtx_cost (PR81288)

[Segher Boessenkool]

The current rs6000 rtx_cost for comparisons against 0 is very high if
TARGET_ISEL && !TARGET_MFCRF, much higher than for reg-reg comparisons,
much higher than a load of 0 and such a reg-reg-comparison.  This leads
to infinite recursion in CSE (see PR81288).

This patch removes the too-high cost, also simplifying this code.

Tested on powerpc64-linux {-m32,-m64}; committing to trunk.  I'll backport
this later this week.


Segher


2017-11-27  Segher Boessenkool  <seg...@kernel.crashing.org>

        PR 81288/target
        * config/rs6000/rs6000.c (rs6000_rtx_costs): Do not handle
        TARGET_ISEL && !TARGET_MFCRF differently.  Simplify code.

---
 gcc/config/rs6000/rs6000.c | 23 ++++++-----------------
 1 file changed, 6 insertions(+), 17 deletions(-)

diff --git a/gcc/config/rs6000/rs6000.c b/gcc/config/rs6000/rs6000.c
index c877d98..7afa4c1 100644
--- a/gcc/config/rs6000/rs6000.c
+++ b/gcc/config/rs6000/rs6000.c
@@ -34980,14 +34980,16 @@ rs6000_rtx_costs (rtx x, machine_mode mode, int 
outer_code,
          *total = COSTS_N_INSNS (1);
          return true;
        }
+      /* FALLTHRU */
+
+    case GT:
+    case LT:
+    case UNORDERED:
       if (outer_code == SET)
        {
          if (XEXP (x, 1) == const0_rtx)
            {
-             if (TARGET_ISEL && !TARGET_MFCRF)
-               *total = COSTS_N_INSNS (8);
-             else
-               *total = COSTS_N_INSNS (2);
+             *total = COSTS_N_INSNS (2);
              return true;
            }
          else
@@ -34996,19 +34998,6 @@ rs6000_rtx_costs (rtx x, machine_mode mode, int 
outer_code,
              return false;
            }
        }
-      /* FALLTHRU */
-
-    case GT:
-    case LT:
-    case UNORDERED:
-      if (outer_code == SET && (XEXP (x, 1) == const0_rtx))
-       {
-         if (TARGET_ISEL && !TARGET_MFCRF)
-           *total = COSTS_N_INSNS (8);
-         else
-           *total = COSTS_N_INSNS (2);
-         return true;
-       }
       /* CC COMPARE.  */
       if (outer_code == COMPARE)
        {
-- 
1.8.3.1