[Bug rtl-optimization/108039] New: Unnecessary extension on riscv-64

[law at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108039

            Bug ID: 108039
           Summary: Unnecessary extension on riscv-64
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: law at gcc dot gnu.org
                CC: rzinsly at ventanamicro dot com
  Target Milestone: ---
            Target: riscv-64

Compile the following code for rv64 with -O2:

typedef signed long int int64_t;
void replace_weaker_arc(int *id1, int *id2, int64_t number)
{
    *id1 = number;
    *id2 = number;
}

We generate:

replace_weaker_arc:
        sext.w  a2,a2
        sw      a2,0(a0)
        sw      a2,0(a1)
        ret


The key insns (from cse1 dump) are:

(insn 8 5 9 2 (set (reg:DI 134 [ _1 ])
        (sign_extend:DI (subreg:SI (reg/v:DI 137 [ number ]) 0))) "j.c":4:10
116 {extendsidi2}
     (nil))
(insn 9 8 10 2 (set (mem:SI (reg/v/f:DI 135 [ id1 ]) [1 *id1_4(D)+0 S4 A32])
        (subreg/s/u:SI (reg:DI 134 [ _1 ]) 0)) "j.c":4:10 178 {*movsi_internal}
     (nil))
(insn 10 9 0 2 (set (mem:SI (reg/v/f:DI 136 [ id2 ]) [1 *id2_6(D)+0 S4 A32])
        (subreg/s/u:SI (reg:DI 134 [ _1 ]) 0)) "j.c":5:10 178 {*movsi_internal}
     (nil))


fwprop tries to propagate insn 8 into insns 9 and 10, but that fails the
complexity check:

cannot propagate from insn 8 into insn 9: would increase complexity of pattern
cannot propagate from insn 8 into insn 10: would increase complexity of pattern

But propagation in this case allows us to eliminate a subreg & extension, so
it's profitable.

I haven't tested it, but something like this captures the RTL propagation
generates and the fact that it should simplify.  We may need to tighten it a
little by verifying modes:
diff --git a/gcc/fwprop.cc b/gcc/fwprop.cc
index fc652ab9a1f..a86e9320908 100644
--- a/gcc/fwprop.cc
+++ b/gcc/fwprop.cc
@@ -258,6 +258,14 @@ fwprop_propagation::classify_result (rtx old_rtx, rtx
new_rtx)
       return CONSTANT | PROFITABLE;
     }

+  /* Allow replacements where we are able to eliminate a
+     (subreg (any_extend (...)).  */
+  if (GET_CODE (old_rtx) == SUBREG
+      && (GET_CODE (SUBREG_REG (old_rtx)) == SIGN_EXTEND
+         || GET_CODE (SUBREG_REG (old_rtx)) == ZERO_EXTEND)
+      && XEXP (SUBREG_REG (old_rtx), 0) == new_rtx)
+    return PROFITABLE;