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arc: Determine a branch target of DBNZ correctly
DBNZ instruction was moved from BRANCH class to a separate one - DBNZ. Thus, it must be processed separately in arc_insn_get_branch_target to correctly determine an offset for a possible branch. The testsuite for DBNZ instruction verifies these cases: 1. Check that dbnz does not branch and falls through if its source register is 0 after decrementing. GDB must successfully break on the following instruction after stepping over. 2. Check that dbnz branches to the target correctly if its source register is not 0 after decrementing - GDB must successfully break on the target instruction if a forward branch is performed after stepping over. 3. The same as point 2 but for a backward branching case. Signed-off-by: Yuriy Kolerov <kolerov93@gmail.com>
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‎gdb/arc-tdep.c‎

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@@ -465,6 +465,16 @@ arc_insn_get_branch_target (const struct arc_instruction &insn)
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instruction, hence last two bits should be truncated. */
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return pcrel_addr + align_down (insn.address, 4);
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}
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/* DBNZ is the only branch instruction that keeps a branch address in
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the second operand. It must be intercepted and treated differently. */
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else if (insn.insn_class == DBNZ)
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{
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CORE_ADDR pcrel_addr = arc_insn_get_operand_value_signed (insn, 1);
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/* Offset is relative to the 4-byte aligned address of the current
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instruction, hence last two bits should be truncated. */
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return pcrel_addr + align_down (insn.address, 4);
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}
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/* B, Bcc, BL, BLcc, LP, LPcc: PC = currentPC + operand. */
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else if (insn.insn_class == BRANCH || insn.insn_class == LOOP)
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{

‎gdb/testsuite/gdb.arch/arc-dbnz.S‎

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; This testcase is part of GDB, the GNU debugger.
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; Copyright 2024 Free Software Foundation, Inc.
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; This program is free software; you can redistribute it and/or modify
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; it under the terms of the GNU General Public License as published by
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; the Free Software Foundation; either version 3 of the License, or
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; (at your option) any later version.
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;
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; This program is distributed in the hope that it will be useful,
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; but WITHOUT ANY WARRANTY; without even the implied warranty of
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; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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; GNU General Public License for more details.
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;
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; You should have received a copy of the GNU General Public License
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; along with this program. If not, see <http://www.gnu.org/licenses/>.
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.section .text
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.align 4
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.global main
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.type main, @function
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main:
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mov r0,1
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dbnz1:
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; r0 == 0 after decrementing. dbnz doesn't do branch.
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dbnz r0,@end
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mov r0,5
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dbnz2:
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; r0 == 3 after decrementing and delay slot. dbnz does branch.
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dbnz.d r0,@dbnz3
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sub r0,r0,1
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dbnz4:
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; r0 == 1 after decrementing. dbnz does branch.
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dbnz r0,@end
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dbnz3:
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; r0 == 2 after decrementing. dbnz does branch.
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dbnz r0,@dbnz4
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end:
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mov r0,0
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j [blink]
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.section .note.GNU-stack,"",@progbits

‎gdb/testsuite/gdb.arch/arc-dbnz.exp‎

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# This testcase is part of GDB, the GNU debugger.
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# Copyright 2024 Free Software Foundation, Inc.
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# This program is free software; you can redistribute it and/or modify
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# it under the terms of the GNU General Public License as published by
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# the Free Software Foundation; either version 3 of the License, or
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# (at your option) any later version.
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#
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# This program is distributed in the hope that it will be useful,
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# but WITHOUT ANY WARRANTY; without even the implied warranty of
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# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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# GNU General Public License for more details.
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#
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# You should have received a copy of the GNU General Public License
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# along with this program. If not, see <http://www.gnu.org/licenses/>.
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# Test dbnz instruction. It decrements its source register operand, and if
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# the result is non-zero it branches to the location defined by a signed
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# half-word displacement operand.
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#
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# It's necessary to verify these cases:
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#
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# 1. Check that dbnz does not branch and falls through if its source
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# register is 0 after decrementing. GDB must successfully break
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# on the following instruction after stepping over.
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# 2. Check that dbnz branches to the target correctly if its source register
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# is not 0 after decrementing - GDB must successfully break on the target
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# instruction if a forward branch is performed after stepping over.
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# 3. The same as point 2 but for a backward branching case.
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require {istarget "arc*-*-*"}
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standard_testfile .S
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if { [prepare_for_testing "failed to prepare" $testfile $srcfile] } {
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return -1
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}
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if ![runto_main] {
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return 0
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}
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gdb_test "break dbnz1" \
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"Breakpoint $decimal at .*" \
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"set breakpoint on the 1st dbnz"
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gdb_test "break dbnz2" \
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"Breakpoint $decimal at .*" \
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"set breakpoint on the 2nd dbnz"
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gdb_test "break dbnz3" \
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"Breakpoint $decimal at .*" \
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"set breakpoint on the 3rd dbnz"
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gdb_test "break dbnz4" \
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"Breakpoint $decimal at .*" \
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"set breakpoint on the 4th dbnz"
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gdb_test "break end" \
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"Breakpoint $decimal at .*" \
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"set breakpoint at the end"
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gdb_test "continue" \
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"Breakpoint $decimal, dbnz1.*dbnz r0,@end" \
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"continue to the 1st dbnz"
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gdb_test "x /i \$pc" \
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"$hex <.*>:\[ \t\]+dbnz\[ \t\]+r0,24.*" \
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"stayng on the 1st dbnz instruction"
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gdb_test "stepi" \
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"mov r0,5" \
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"step over the 1st dbnz, branch is not taken"
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gdb_test "stepi" \
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"Breakpoint $decimal, dbnz2.*dbnz\\.d r0,@dbnz3" \
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"step over r0 initialization, staying on the 2nd dbnz"
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# Linux steps over delay slot after "stepi", but stubs with hardware stepping
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# like nSIM's stub may step right on delay slot. Thus use "continue" instead of
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# "stepi" to make this test work for all platforms.
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gdb_test "continue" \
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"Breakpoint $decimal, dbnz3.*dbnz r0,@dbnz4" \
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"step over the 2nd dbnz, branch is taken, staying on the 3rd dbnz"
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gdb_test "stepi" \
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"Breakpoint $decimal, dbnz4.*dbnz r0,@end" \
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"step over the 3rd dbnz, branch is taken, staying on the 4th dbnz"
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gdb_test "stepi" \
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"Breakpoint $decimal, end.*mov r0,0" \
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"step over the 4th dbnz, branch is taken, staying on the epilogue"
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gdb_test "info register r0" \
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"r0\[ \t\]+0x1\[ \t\]+1" \
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"r0 contains 1 after all dbnz instructions"

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