[gcc(refs/vendors/riscv/heads/gcc-15-with-riscv-opts)] ifcvt: Handle lowpart subregs if noce_emit_cmove fails in noce_try_cond_zero_arith [PR123308]

[Jeff Law]

https://gcc.gnu.org/g:89ed8490301616eafdd2abe4ef759060cbc8bf59

commit 89ed8490301616eafdd2abe4ef759060cbc8bf59
Author: Andrew Pinski <andrew.pinski@oss.qualcomm.com>
Date:   Fri Dec 26 12:07:26 2025 -0800

    ifcvt: Handle lowpart subregs if noce_emit_cmove fails in noce_try_cond_zero_arith [PR123308]
    
    This fixes up a missed optimization regression for riscv in ifcvt after r16-6350-g9e61a171244110.
    The problem is noce_emit_cmove will fail for QImode. This can show up when dealing with shifts
    and the right hand side is `(subreg:QI (reg:DI) lowpart)`. Trying first for the subreg mode
    fails so the need to try to unwrap the subreg and try for the full mode.
    
    This fixes test_ShiftLeft_eqz in gcc.target/riscv/zicond_ifcvt_opt.c.
    
    Bootstrapped and tested on x86_64-linux-gnu.
    
            PR rtl-optimization/123308
    gcc/ChangeLog:
    
            * ifcvt.cc (noce_try_cond_zero_arith): If noce_emit_cmove fails
            for a lowpart subreg case, then try the full reg cmove and
            take the lowpart subreg afterwards.
    
    Signed-off-by: Andrew Pinski <andrew.pinski@oss.qualcomm.com>
    (cherry picked from commit 92f2a86794ad88d773b102c753e13be1b6f05ff9)

Diff:
---
 gcc/ifcvt.cc | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/gcc/ifcvt.cc b/gcc/ifcvt.cc
index 1f6538beb791..e2f98fbd5fbc 100644
--- a/gcc/ifcvt.cc
+++ b/gcc/ifcvt.cc
@@ -3210,6 +3210,28 @@ noce_try_cond_zero_arith (struct noce_if_info *if_info)
 			    XEXP (cond, 0), XEXP (cond, 1),
 			    op != AND ? XEXP (a, 1) : const0_rtx,
 			    op != AND ? const0_rtx : XEXP (a, 0));
+  if (!target)
+    {
+      end_sequence ();
+      rtx tmp = XEXP (a, op != AND);
+      /* If the cmove fails and this was a lowpart subreg,
+	 then try the reg part and then putting back the lowpart
+	 afterwards.  */
+      if (GET_CODE (tmp) != SUBREG  || !subreg_lowpart_p (tmp))
+	return false;
+      start_sequence ();
+
+      tmp = SUBREG_REG (tmp);
+      target = gen_reg_rtx (GET_MODE (tmp));
+      target = noce_emit_cmove (if_info, target, code,
+				XEXP (cond, 0), XEXP (cond, 1),
+				op != AND ? tmp : const0_rtx,
+				op != AND ? const0_rtx : tmp);
+      if (!target)
+	goto end_seq_n_fail;
+      target = rtl_hooks.gen_lowpart_no_emit (GET_MODE (XEXP (a, op != AND)), target);
+      gcc_assert (target);
+    }
   if (!target)
     goto end_seq_n_fail;