gdb/aarch64: Disable displaced single-step for MOPS instructions

The AArch64 MOPS (Memory Operation) instructions provide a standardised
instruction sequence to perform a memset, memcpy or memmove.  A sequence is
always composed of three instructions: a prologue instruction, a main
instruction and an epilogue instruction.  As an illustration, here are the
implementations of these memory operations in glibc 2.39:

  (gdb) disassemble/r
  Dump of assembler code for function __memset_mops:
  => 0x0000fffff7e8d780 <+0>:     d503201f        nop
     0x0000fffff7e8d784 <+4>:     aa0003e3        mov     x3, x0
     0x0000fffff7e8d788 <+8>:     19c10443        setp    [x3]!, x2!, x1
     0x0000fffff7e8d78c <+12>:    19c14443        setm    [x3]!, x2!, x1
     0x0000fffff7e8d790 <+16>:    19c18443        sete    [x3]!, x2!, x1
     0x0000fffff7e8d794 <+20>:    d65f03c0        ret
  End of assembler dump.

  (gdb) disassemble/r
  Dump of assembler code for function __memcpy_mops:
  => 0x0000fffff7e8c580 <+0>:     d503201f        nop
     0x0000fffff7e8c584 <+4>:     aa0003e3        mov     x3, x0
     0x0000fffff7e8c588 <+8>:     19010443        cpyfp   [x3]!, [x1]!, x2!
     0x0000fffff7e8c58c <+12>:    19410443        cpyfm   [x3]!, [x1]!, x2!
     0x0000fffff7e8c590 <+16>:    19810443        cpyfe   [x3]!, [x1]!, x2!
     0x0000fffff7e8c594 <+20>:    d65f03c0        ret
  End of assembler dump.

  (gdb) disassemble/r
  Dump of assembler code for function __memmove_mops:
  => 0x0000fffff7e8d180 <+0>:     d503201f        nop
     0x0000fffff7e8d184 <+4>:     aa0003e3        mov     x3, x0
     0x0000fffff7e8d188 <+8>:     1d010443        cpyp    [x3]!, [x1]!, x2!
     0x0000fffff7e8d18c <+12>:    1d410443        cpym    [x3]!, [x1]!, x2!
     0x0000fffff7e8d190 <+16>:    1d810443        cpye    [x3]!, [x1]!, x2!
     0x0000fffff7e8d194 <+20>:    d65f03c0        ret
  End of assembler dump.

The Arm Architecture Reference Manual says that "the prologue, main, and
epilogue instructions are expected to be run in succession and to appear
consecutively in memory".  Therefore this patch disables displaced stepping
on them.

The testcase verifies that MOPS sequences are correctly single-stepped.

PR tdep/31666
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31666
Approved-By: Luis Machado <luis.machado@arm.com>
Tested-By: Luis Machado <luis.machado@arm.com>

4 files changed, 275 insertions, 3 deletions

diff --git a/gdb/aarch64-tdep.c b/gdb/aarch64-tdep.c
index 8d0553f..05ecd42 100644
--- a/gdb/aarch64-tdep.c
+++ b/gdb/aarch64-tdep.c
@@ -3808,10 +3808,12 @@ aarch64_displaced_step_copy_insn (struct gdbarch *gdbarch,
   if (aarch64_decode_insn (insn, &inst, 1, NULL) != 0)
     return NULL;
 
-  /* Look for a Load Exclusive instruction which begins the sequence.  */
-  if (inst.opcode->iclass == ldstexcl && bit (insn, 22))
+  /* Look for a Load Exclusive instruction which begins the sequence,
+     or for a MOPS instruction.  */
+  if ((inst.opcode->iclass == ldstexcl && bit (insn, 22))
+      || AARCH64_CPU_HAS_FEATURE (*inst.opcode->avariant, MOPS))
     {
-      /* We can't displaced step atomic sequences.  */
+      /* We can't displaced step atomic sequences nor MOPS instructions.  */
       return NULL;
     }
 
diff --git a/gdb/testsuite/gdb.arch/aarch64-mops-single-step.c b/gdb/testsuite/gdb.arch/aarch64-mops-single-step.c
new file mode 100644
index 0000000..4a27867
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/aarch64-mops-single-step.c
@@ -0,0 +1,73 @@
+/* This file is part of GDB, the GNU debugger.
+
+   Copyright 2024 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+#define TEST_STRING "Just a test string."
+#define BUF_SIZE sizeof(TEST_STRING)
+
+int
+main (void)
+{
+  char source[BUF_SIZE] = TEST_STRING;
+  char dest[BUF_SIZE];
+  char *p, *q;
+  long size, zero;
+
+  /* Note: The prfm instruction in the asm statements below is there just
+     to allow the testcase to recognize when the PC is at the instruction
+     right after the MOPS sequence.  */
+
+  p = dest;
+  size = sizeof (dest);
+  zero = 0;
+  /* Break memset.  */
+  /* memset implemented in MOPS instructions.  */
+  __asm__ volatile ("setp [%0]!, %1!, %2\n\t"
+		    "setm [%0]!, %1!, %2\n\t"
+		    "sete [%0]!, %1!, %2\n\t"
+		    "prfm pldl3keep, [%0, #0]\n\t"
+		    : "+&r"(p), "+&r"(size)
+		    : "r"(zero)
+		    : "memory");
+
+  p = dest;
+  q = source;
+  size = sizeof (dest);
+  /* Break memcpy.  */
+  /* memcpy implemented in MOPS instructions.  */
+  __asm__ volatile ("cpyfp [%0]!, [%1]!, %2!\n\t"
+		    "cpyfm [%0]!, [%1]!, %2!\n\t"
+		    "cpyfe [%0]!, [%1]!, %2!\n\t"
+		    "prfm pldl3keep, [%0, #0]\n\t"
+		    : "+&r" (p), "+&r" (q), "+&r" (size)
+		    :
+		    : "memory");
+
+  p = dest;
+  q = source;
+  size = sizeof (dest);
+  /* Break memmove.  */
+  /* memmove implemented in MOPS instructions.  */
+  __asm__ volatile ("cpyp [%0]!, [%1]!, %2!\n\t"
+		    "cpym [%0]!, [%1]!, %2!\n\t"
+		    "cpye [%0]!, [%1]!, %2!\n\t"
+		    "prfm pldl3keep, [%0, #0]\n\t"
+		    : "+&r" (p), "+&r" (q), "+&r" (size)
+		    :
+		    : "memory");
+
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.arch/aarch64-mops-single-step.exp b/gdb/testsuite/gdb.arch/aarch64-mops-single-step.exp
new file mode 100644
index 0000000..2cb5fdc
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/aarch64-mops-single-step.exp
@@ -0,0 +1,98 @@
+# Copyright 2024 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+# This file is part of the GDB testsuite.
+
+# Test single stepping through MOPS (memory operations) instruction sequences.
+
+require allow_aarch64_mops_tests
+
+standard_testfile
+if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile} \
+	  [list debug additional_flags=-march=armv9.3-a]] } {
+    return -1
+}
+
+# If the inferior is rescheduled to another CPU while a main or epilogue
+# instruction is executed, the OS resets the inferior back to the prologue
+# instruction, so we need to allow for that possibility.
+proc step_through_sequence { prefix } {
+    set count 0
+
+    while { [is_at_instruction ${prefix}p] == 1 && $count < 50 } {
+	incr count
+
+	# The stepi output isn't useful to detect whether we stepped over
+	# the instruction.
+	gdb_test "stepi" "\[^\r\n\]+" "step over ${prefix}p"
+	if { [is_at_instruction ${prefix}m] == 1 } {
+	    pass "stepped over ${prefix}p"
+	} else {
+	    fail "stepped over ${prefix}e"
+	    return 0
+	}
+
+	gdb_test "stepi" "\[^\r\n\]+" "step over ${prefix}m"
+	if { [is_at_instruction ${prefix}e] == 1 } {
+	    pass "stepped over ${prefix}m"
+	} elseif { [is_at_instruction ${prefix}p] == 1 } {
+	    # The inferior was rescheduled to another CPU.
+	    pass "${prefix}m: reset back to prologue"
+	    continue
+	} else {
+	    fail "stepped over ${prefix}m"
+	    return 0
+	}
+
+	gdb_test "stepi" "\[^\r\n\]+" "step over ${prefix}e"
+	if { [is_at_instruction prfm] == 1 } {
+	    pass "stepped over ${prefix}e"
+	    return 1
+	} elseif { [is_at_instruction ${prefix}p] == 1 } {
+	    # The inferior was rescheduled to another CPU.
+	    pass "${prefix}e: reset back to prologue"
+	    continue
+	}
+    }
+
+    fail "step through $prefix sequence"
+    return 0
+}
+
+if ![runto_main] {
+    return -1
+}
+
+gdb_breakpoint ${srcfile}:[gdb_get_line_number "Break memset"]
+gdb_breakpoint ${srcfile}:[gdb_get_line_number "Break memcpy"]
+gdb_breakpoint ${srcfile}:[gdb_get_line_number "Break memmove"]
+
+gdb_continue_to_breakpoint "memset breakpoint"
+
+if { [arrive_at_instruction setp] } {
+    step_through_sequence set
+}
+
+gdb_continue_to_breakpoint "memcpy breakpoint"
+
+if { [arrive_at_instruction cpyfp] } {
+    step_through_sequence cpyf
+}
+
+gdb_continue_to_breakpoint "memmove breakpoint"
+
+if { [arrive_at_instruction cpyp] } {
+    step_through_sequence cpy
+}
diff --git a/gdb/testsuite/lib/gdb.exp b/gdb/testsuite/lib/gdb.exp
index 5aeaa8d..590cd45 100644
--- a/gdb/testsuite/lib/gdb.exp
+++ b/gdb/testsuite/lib/gdb.exp
@@ -848,6 +848,44 @@ proc gdb_continue_to_breakpoint {name {location_pattern .*}} {
     }]
 }
 
+# Check whether GDB is stopped at the given instruction.
+# INSTRUCTION should be just its mnemonic, without any arguments.
+
+proc is_at_instruction { instruction } {
+    global gdb_prompt hex
+
+    set test "pc points to $instruction"
+    gdb_test_multiple {x/i $pc} $test {
+	-re -wrap "=> $hex \[^\r\n\]+:\t$instruction\t\[^\r\n\]+" {
+	    return 1
+	}
+	-re "\r\n$gdb_prompt $" {
+	    return 0
+	}
+    }
+
+    return 0
+}
+
+# Single-steps GDB until it arrives at the given instruction.
+# INSTRUCTION should be just its mnemonic, without any arguments.
+
+proc arrive_at_instruction { instruction } {
+    set count 0
+
+    while { [is_at_instruction $instruction] != 1 } {
+	gdb_test -nopass "stepi" "\[^\r\n\]+" \
+	    "stepi #$count to reach $instruction"
+	incr count
+
+	if { $count > 50 } {
+	    fail "didn't reach $instruction"
+	    return 0
+	}
+    }
+
+    return 1
+}
 
 # gdb_internal_error_resync:
 #
@@ -4496,6 +4534,67 @@ proc aarch64_supports_sme_svl { length } {
     return 1
 }
 
+# Run a test on the target to see if it supports AArch64 MOPS (Memory
+# Operations) extensions.  Return 1 if so, 0 if it does not.  Note this
+# causes a restart of GDB.
+
+gdb_caching_proc allow_aarch64_mops_tests {} {
+    global srcdir subdir gdb_prompt inferior_exited_re
+
+    set me "allow_aarch64_mops_tests"
+
+    if { ![is_aarch64_target]} {
+	return 0
+    }
+
+    # ARMv9.3-A contains the MOPS extension.  The test program doesn't use it,
+    # but take the opportunity to check whether the toolchain knows about MOPS.
+    set compile_flags "{additional_flags=-march=armv9.3-a}"
+
+    # Compile a program that tests the MOPS feature.
+    set src {
+	#include <stdbool.h>
+	#include <sys/auxv.h>
+
+	#ifndef HWCAP2_MOPS
+	#define HWCAP2_MOPS (1UL << 43)
+	#endif
+
+	int main() {
+	    bool mops_supported = getauxval (AT_HWCAP2) & HWCAP2_MOPS;
+
+	    return !mops_supported;
+	}
+    }
+
+    if {![gdb_simple_compile $me $src executable $compile_flags]} {
+	return 0
+    }
+
+    # Compilation succeeded so now run it via gdb.
+    clean_restart $obj
+    gdb_run_cmd
+    gdb_expect {
+	-re ".*$inferior_exited_re with code 01.*${gdb_prompt} $" {
+	    verbose -log "\n$me mops support not detected"
+	    set allow_mops_tests 0
+	}
+	-re ".*$inferior_exited_re normally.*${gdb_prompt} $" {
+	    verbose -log "\n$me: mops support detected"
+	    set allow_mops_tests 1
+	}
+	default {
+	  warning "\n$me: default case taken"
+	    set allow_mops_tests 0
+	}
+    }
+    gdb_exit
+    remote_file build delete $obj
+
+    verbose "$me:  returning $allow_mops_tests" 2
+    return $allow_mops_tests
+}
+
 # A helper that compiles a test case to see if __int128 is supported.
 proc gdb_int128_helper {lang} {
     return [gdb_can_simple_compile "i128-for-$lang" {