[gcc r13-6294] xtensa: fix PR target/108876

[gcc r13-6294] xtensa: fix PR target/108876

[Max Filippov]

https://gcc.gnu.org/g:782e442e4f3b88937a725a01245dd749f3f893b9

commit r13-6294-g782e442e4f3b88937a725a01245dd749f3f893b9
Author: Max Filippov <jcmvbkbc@gmail.com>
Date:   Wed Feb 22 22:08:21 2023 -0800

    xtensa: fix PR target/108876
    
    In commit b2ef02e8cbbaf95fee98be255f697f47193960ec, the sibling call
    insn included (use (reg:SI A0_REG)) to fix the problem, which added
    a USE chain unconditionally to the data flow of register A0 during
    the sibling call.
    
    As a result, df_regs_ever_live_p (A0_REG) returns true, so even if
    register A0 is not used outside of the sibling call insn, saves and
    restores to stack slots are emitted in pro/epilogue, and finally
    code size increases.
    (This is why I never included (use A0) in sibling calls)
    
        /* example */
        extern int foo(int);
        int test(int a) {
          return foo(a * 3 + 1);
        }
    
    ;; before
        test:
            addi    sp, sp, -16     ;; unneeded stack frame allocation (induced)
            s32i.n  a0, sp, 12      ;; unneeded saving of register A0
            l32i.n  a0, sp, 12      ;; unneeded restoration of register A0
            addx2   a2, a2, a2
            addi.n  a2, a2, 1
            addi    sp, sp, 16      ;; unneeded stack frame freeing (induced)
            j.l     foo, a9         ;; sibling call (truly needs register A0)
    
    The essential cause is that we emit (use A0) *before* the insns that
    does the stack pointer adjustment during epilogue expansion, so the
    liveness of register A0 ends early, so register A0 is reused afterwards.
    
    This patch fixes the problem and avoids such regression by doing the
    emit of (use A0) in the sibling call epilogue expansion at the end.
    
    ;; after
    test:
            addx2   a2, a2, a2
            addi.n  a2, a2, 1
            j.l     foo, a9
    
    >From RTL-pass "315r.rnreg" by
    "gfortran -O3 -funroll-loops -mabi=call0 -S -da gcc-gnu/gcc/testsuite/gfortran.dg/allocate_with_source_5.f90":
    
        ;; Function selector_init (__selectors_MOD_selector_init, funcdef_no=2, decl_uid=987, cgraph_uid=3, symbol_order=4)
        ...
        (insn 3807 3806 3808 121 (set (reg:SI 15 a15)
                (mem/c:SI (plus:SI (reg/f:SI 1 sp)
                        (const_int 268 [0x10c])) [31  S4 A32])) "gcc-gnu/gcc/testsuite/gfortran.dg/allocate_with_source_5.f90":35:30 53 {movsi_internal}
             (nil))
        (insn 3808 3807 3809 121 (set (reg:SI 7 a7)
                (const_int 288 [0x120])) "gcc-gnu/gcc/testsuite/gfortran.dg/allocate_with_source_5.f90":35:30 53 {movsi_internal}
             (nil))
        (insn 3809 3808 3810 121 (set (reg/f:SI 1 sp)
                (plus:SI (reg/f:SI 1 sp)
                    (reg:SI 7 a7))) "gcc-gnu/gcc/testsuite/gfortran.dg/allocate_with_source_5.f90":35:30 1 {addsi3}
             (expr_list:REG_DEAD (reg:SI 9 a9)
                (nil)))
        (insn 3810 3809 721 121 (use (reg:SI 0 a0)) "gcc-gnu/gcc/testsuite/gfortran.dg/allocate_with_source_5.f90":35:30 -1
             (expr_list:REG_DEAD (reg:SI 0 a0)
                (nil)))
        (call_insn/j 721 3810 722 121 (call (mem:SI (symbol_ref:SI ("free") [flags 0x41]  <function_decl 0x7f7c885f6000 __builtin_free>) [0 __builtin_free S4 A32])
                (const_int 0 [0])) "gcc-gnu/gcc/testsuite/gfortran.dg/allocate_with_source_5.f90":35:30 discrim 1 106 {sibcall_internal}
             (expr_list:REG_DEAD (reg:SI 2 a2)
                (expr_list:REG_CALL_DECL (symbol_ref:SI ("free") [flags 0x41]  <function_decl 0x7f7c885f6000 __builtin_free>)
                    (expr_list:REG_EH_REGION (const_int 0 [0])
                        (nil))))
            (expr_list:SI (use (reg:SI 2 a2))
                (nil)))
    
    (IMHO the "rnreg" pass doesn't take REG_ALLOC_ORDER into account;
    it just seems to allocate registers in fixed_regs index order,
    which may have hurt register A0 that became allocatable in the recent
    patch)
    
    gcc/ChangeLog:
            PR target/108876
    
            * config/xtensa/xtensa.cc (xtensa_expand_epilogue):
            Emit (use (reg:SI A0_REG)) at the end in the sibling call
            (i.e. the same place as (return) in the normal call).

Diff:
---
 gcc/config/xtensa/xtensa.cc | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/gcc/config/xtensa/xtensa.cc b/gcc/config/xtensa/xtensa.cc
index d0320efe21d..b80eef5c19e 100644
--- a/gcc/config/xtensa/xtensa.cc
+++ b/gcc/config/xtensa/xtensa.cc
@@ -3548,8 +3548,6 @@ xtensa_expand_epilogue (bool sibcall_p)
 			      gen_frame_mem (SImode, x));
 	    }
 	}
-      if (sibcall_p)
-	emit_use (gen_rtx_REG (SImode, A0_REG));
 
       if (cfun->machine->current_frame_size > 0)
 	{
@@ -3575,7 +3573,9 @@ xtensa_expand_epilogue (bool sibcall_p)
 				  EH_RETURN_STACKADJ_RTX));
     }
   cfun->machine->epilogue_done = true;
-  if (!sibcall_p)
+  if (sibcall_p)
+    emit_use (gen_rtx_REG (SImode, A0_REG));
+  else
     emit_jump_insn (gen_return ());
 }