[wide-int] Fix mode choice in convert_modes

[Richard Sandiford]

Enabling the CONST_INT assert showed that this branch-local code was
passing the wrong mode to std::make_pair.  The mode of X is OLDMODE,
and we're supposed to be converting it to MODE.

In the case where OLDMODE is VOIDmode, I think we have to assume
that every bit of the input is significant.

Tested on x86_64-linux-gnu.  OK to install?

This and the other patches that I just committed to trunk are enough
for a bootstrap and regression test to pass on x86_64-linux-gnu with
the assertion for a canonical CONST_INT enabled.  I think it would be
better to leave the assertion out until after the merge though,
so we can deal with any fallout separately.

Thanks,
Richard


Index: gcc/expr.c
===================================================================
--- gcc/expr.c	2013-09-09 11:32:33.734617409 +0100
+++ gcc/expr.c	2013-09-09 11:45:27.381001353 +0100
@@ -714,13 +714,14 @@ convert_modes (enum machine_mode mode, e
       && GET_MODE_CLASS (mode) == MODE_INT
       && (oldmode == VOIDmode || GET_MODE_CLASS (oldmode) == MODE_INT))
     {
-      wide_int w = std::make_pair (x, mode);
       /* If the caller did not tell us the old mode, then there is
-	 not much to do with respect to canonization.  */
-      if (oldmode != VOIDmode
-	  && GET_MODE_PRECISION (mode) > GET_MODE_PRECISION (oldmode))
-	w = wi::ext (w, GET_MODE_PRECISION (oldmode),
-		     unsignedp ? UNSIGNED : SIGNED);
+	 not much to do with respect to canonization.  We have to assume
+	 that all the bits are significant.  */
+      if (oldmode == VOIDmode)
+	oldmode = MAX_MODE_INT;
+      wide_int w = wide_int::from (std::make_pair (x, oldmode),
+				   GET_MODE_PRECISION (mode),
+				   unsignedp ? UNSIGNED : SIGNED);
       return immed_wide_int_const (w, mode);
     }