(force_to_mode...

(force_to_mode, case xSHIFT): Don't narrow the mode unless we can be
sure that the shift count is smaller than the size of the mode.

From-SVN: r4050

diff --git a/gcc/combine.c b/gcc/combine.c
index 6993356eb07472d017d743e2e767ad284d5e1139..11dec08d7cbc0db4bb8603efd6e7ce89d9b57e7c 100644 (file)
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -5342,9 +5342,21 @@ force_to_mode (x, mode, bits, reg)
     case ASHIFT:
     case LSHIFT:
       /* For left shifts, do the same, but just for the first operand.
-        If the shift count is a constant, we need even fewer bits of the
-        first operand.  */
-
+        However, we cannot do anything with shifts where we cannot
+        guarantee that the counts are smaller than the size of the mode
+        because such a count will have a different meaning in a
+        wider mode.
+
+        If we can narrow the shift and know the count, we need even fewer
+        bits of the first operand.  */
+
+      if (! (GET_CODE (XEXP (x, 1)) == CONST_INT
+            && INTVAL (XEXP (x, 1)) < GET_MODE_BITSIZE (mode))
+         && ! (GET_MODE (XEXP (x, 1)) != VOIDmode
+               && (nonzero_bits (XEXP (x, 1), GET_MODE (XEXP (x, 1)))
+                   < (unsigned) GET_MODE_BITSIZE (mode))))
+       break;
+       
       if (GET_CODE (XEXP (x, 1)) == CONST_INT && INTVAL (XEXP (x, 1)) < bits)
        bits -= INTVAL (XEXP (x, 1));
 
@@ -5364,8 +5376,8 @@ force_to_mode (x, mode, bits, reg)
 
     case LSHIFTRT:
       /* Here we can only do something if the shift count is a constant and
-        the count plus BITS is no larger than the width of MODE, we can do
-        the shift in MODE.  */
+        the count plus BITS is no larger than the width of MODE.  In that
+        case, we can do the shift in MODE.  */
 
       if (GET_CODE (XEXP (x, 1)) == CONST_INT
          && INTVAL (XEXP (x, 1)) + bits <= GET_MODE_BITSIZE (mode))