[Bug target/63321] [SH] Unused T bit result of shll / shlr insns

["olegendo at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63321

Oleg Endo <olegendo at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2014-12-02
                 CC|                            |segher at gcc dot gnu.org
     Ever confirmed|0                           |1

--- Comment #1 from Oleg Endo <olegendo at gcc dot gnu.org> ---
Combine recently received some updates which improve handling of multiple-set
parallel insns.  Applying the following:

Index: gcc/config/sh/sh.md
===================================================================
--- gcc/config/sh/sh.md    (revision 218250)
+++ gcc/config/sh/sh.md    (working copy)
@@ -5156,6 +5156,12 @@
       DONE;
     }

+  if (operands[2] == const1_rtx)
+    {
+      emit_insn (gen_shlr (operands[0], operands[1]));
+      DONE;
+    }
+
   /* If the lshrsi3_* insn is going to clobber the T_REG it must be
      expanded here.  */
   if (CONST_INT_P (operands[2])


will always expand the multiple-set shlr insn and combine will be able to
utilize this.  The test case

void test2_1 (unsigned int x, unsigned int* y)
{
  y[0] = x >> 1;
  y[1] = x & 1;
}

will compile to the desired sequence:
        shlr    r4
        movt    r1
        mov.l   r4,@r5
        rts
        mov.l   r1,@(4,r5)


However, in the context of e.g. pointer tagging use cases, the tag bits are
usually used with conditional branches:

void test2_2 (unsigned int x, unsigned int* y)
{
  unsigned int xx = x >> 1;
  unsigned int p = x & 1;
  if (p != 0)
    foo (xx);
}

Combine can't handle this, because the shift and test insns end up in different
basic blocks.  Moreover, in order to utilize the shlr insn, the branch
condition needs to be inverted.  This could be done by emitting a movt-tst
sequence and let the sh_treg_combine pass optimize it away by inverting the
branch condition.