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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Prologue computation is incorrect for single-line functions compiled by GCC"
href="https://bugs.llvm.org/show_bug.cgi?id=43421">43421</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Prologue computation is incorrect for single-line functions compiled by GCC
</td>
</tr>
<tr>
<th>Product</th>
<td>lldb
</td>
</tr>
<tr>
<th>Version</th>
<td>6.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>All Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>lldb-dev@lists.llvm.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>brettw@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>jdevlieghere@apple.com, llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>I compiled this function in g++ (Debian 8.3.0-6) 8.3.0 with the command "gcc -g
eraseme.cc":
#include <stdio.h>
void PrologueTest() { int a; scanf("%d", &a); printf("Scanned %d\n", a);
printf("END\n");
}
int main(int argc, char **argv) {
PrologueTest();
return 0;
}
GCC doesn't seem to generate DWARF prologue_end annotations, at least in my
version:
Address Line Column File ISA Discriminator Flags
------------------ ------ ------ ------ --- ------------- -------------
0x0000000000001155 3 21 1 0 0 is_stmt
0x000000000000115d 3 35 1 0 0 is_stmt
0x0000000000001175 3 53 1 0 0 is_stmt
0x000000000000118b 4 9 1 0 0 is_stmt
0x0000000000001197 5 1 1 0 0 is_stmt
0x000000000000119a 7 33 1 0 0 is_stmt
0x00000000000011a9 8 15 1 0 0 is_stmt
0x00000000000011ae 9 10 1 0 0 is_stmt
0x00000000000011b3 10 1 1 0 0 is_stmt
0x00000000000011b5 10 1 1 0 0 is_stmt
end_sequence
If I set a breakpoint on PrologueTest:
(lldb) b PrologueTest
Breakpoint 1: where = a.out`PrologueTest() + 54 at eraseme.cc:4, address =
0x000000000000118b
The LLDB code in Function::GetPrologueByteSize() notices there's no prologue
end markers and hits the condition commented:
// Check the first few instructions and look for one that has a line
// number that's different than the first entry.
This then slides the breakpoint address to the next line which is line 4. Note
the address above (0x118b) corresponds to the last "puts" call:
(lldb) disassemble -name PrologueTest
a.out`PrologueTest:
a.out[0x1155] <+0>: pushq %rbp
a.out[0x1156] <+1>: movq %rsp, %rbp
a.out[0x1159] <+4>: subq $0x10, %rsp
a.out[0x115d] <+8>: leaq -0x4(%rbp), %rax
a.out[0x1161] <+12>: movq %rax, %rsi
a.out[0x1164] <+15>: leaq 0xe99(%rip), %rdi
a.out[0x116b] <+22>: movl $0x0, %eax
a.out[0x1170] <+27>: callq 0x1050 ; symbol stub for:
scanf
a.out[0x1175] <+32>: movl -0x4(%rbp), %eax
a.out[0x1178] <+35>: movl %eax, %esi
a.out[0x117a] <+37>: leaq 0xe86(%rip), %rdi
a.out[0x1181] <+44>: movl $0x0, %eax
a.out[0x1186] <+49>: callq 0x1040 ; symbol stub for:
printf
a.out[0x118b] <+54>: leaq 0xe81(%rip), %rdi
a.out[0x1192] <+61>: callq 0x1030 ; symbol stub for: puts
a.out[0x1197] <+66>: nop
a.out[0x1198] <+67>: leave
a.out[0x1199] <+68>: retq
This is not what you expect when you set a breakpoint on a function. Running
the program validates this theory.
GDB seems to use the first line table *entry* as the prologue, giving 0x115d as
the address of the breakpoint. This behaves as expected.</pre>
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