[gcc/devel/c++-modules] middle-end: Remove truly_noop_truncation check from convert.c

[Nathan Sidwell]

https://gcc.gnu.org/g:b8697d0cd06da3375063295fe7161671e0904459

commit b8697d0cd06da3375063295fe7161671e0904459
Author: Roger Sayle <roger@nextmovesoftware.com>
Date:   Mon Jul 13 09:49:34 2020 +0100

    middle-end: Remove truly_noop_truncation check from convert.c
    
    This patch eliminates a check of targetm.truly_noop_truncation from
    the early middle-end, where the gimple/generic being generated by
    GCC's front-ends is being inappropriately influenced by the target's
    TRULY_NOOP_TRUNCATION.  The (recent) intention of TRULY_NOOP_TRUNCATION
    is to indicate that a backend requires explicit truncation instructions
    rather than using SUBREGs to perform truncations.  A long standing
    (and probably unintentional) side-effect has been that this setting
    also controls whether the middle-end narrows integer operations at
    the tree-level.  Understandably, GCC and its testsuite assume that
    GIMPLE and GENERIC behave consistently across platforms, and alas
    defining TRULY_NOOP_TRUNCATION away from the default triggers several
    regressions (including gcc.dg/fold-rotate-1.c).
    
    2020-07-13  Roger Sayle  <roger@nextmovesoftware.com>
    
    gcc/ChangeLog
            * convert.c (convert_to_integer_1): Narrow integer operations
            even on targets that require explicit truncation instructions.

Diff:
---
 gcc/convert.c | 14 ++++----------
 1 file changed, 4 insertions(+), 10 deletions(-)

diff --git a/gcc/convert.c b/gcc/convert.c
index 42509c518a9..292c5133974 100644
--- a/gcc/convert.c
+++ b/gcc/convert.c
@@ -836,16 +836,10 @@ convert_to_integer_1 (tree type, tree expr, bool dofold)
 		  && POINTER_TYPE_P (TREE_TYPE (TREE_OPERAND (arg1, 0))))
 		break;
 
-	      if (outprec >= BITS_PER_WORD
-		  || targetm.truly_noop_truncation (outprec, inprec)
-		  || inprec > TYPE_PRECISION (TREE_TYPE (arg0))
-		  || inprec > TYPE_PRECISION (TREE_TYPE (arg1)))
-		{
-		  tree tem = do_narrow (loc, ex_form, type, arg0, arg1,
-					expr, inprec, outprec, dofold);
-		  if (tem)
-		    return tem;
-		}
+	      tree tem = do_narrow (loc, ex_form, type, arg0, arg1,
+				    expr, inprec, outprec, dofold);
+	      if (tem)
+		return tem;
 	    }
 	    break;