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<head>
<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - LTO optimization causes clang to produce incorrect code"
href="https://bugs.llvm.org/show_bug.cgi?id=34132">34132</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>LTO optimization causes clang to produce incorrect code
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Windows NT
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>-New Bugs
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>konstantin.belochapka@sony.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>source.c:
#include <stdio.h>
struct {
unsigned char a[4];
unsigned char b[4];
} s;
static void init_arr(volatile void *data, unsigned char size) {
unsigned char *bytes = (unsigned char *)data;
for (unsigned char i = 0; i != size; ++i) {
bytes[i] = i;
}
}
int main() {
init_arr(&s, sizeof(s) );
printf("a:%x %x %x %x\n", s.a[0], s.a[1], s.a[2], s.a[3]);
printf("b:%x %x %x %x\n", s.b[0], s.b[1], s.b[2], s.b[3]);
}
clang -Oz -flto source.c
or
clang -O0 -flto source.c
or
clang -O1 -flto source.c
<main>:
xor %eax,%eax
lea 0x7fcb(%rip),%rcx # 80a8 <__preinit_array_end>
jmp e5 <__start__Zdynsym+0x5>
mov %al,(%rax,%rcx,1)
inc %rax
cmp $0x8,%rax
jne df <__dynstr_end+0x6>
push %rbp
mov %rsp,%rbp
movzbl 0x7fb2(%rip),%esi # 80a8 <__preinit_array_end>
movzbl 0x7fac(%rip),%edx # 80a9 <__preinit_array_end+0x1>
movzbl 0x7fa6(%rip),%ecx # 80aa <__preinit_array_end+0x2>
movzbl 0x7f9f(%rip),%r8d # 80ab <__preinit_array_end+0x3>
lea 0xdd(%rip),%rdi # 1f0 <.L.str>
xor %eax,%eax
callq 1a0 </PLprintf>
lea 0xde(%rip),%rdi # 1ff <.L.str.1>
xor %esi,%esi
xor %edx,%edx
xor %ecx,%ecx
xor %r8d,%r8d
xor %eax,%eax
callq 1a0 </PLprintf>
xor %eax,%eax
pop %rbp
retq
With LTO optimization clang assumes that s.b[0] == s.b[1] == s.b[2] == s.b[3]
== 0 and generates all zero values for the second printf().
With optimization level -O2 or -O3 and -flto, clang produces correct code:
<main>:
push %rbp
mov %rsp,%rbp
lea 0xf5(%rip),%rdi # 1e0 <.L.str>
mov $0x0,%esi
mov $0x1,%edx
mov $0x2,%ecx
mov $0x3,%r8d
xor %eax,%eax
callq 190 </PLprintf>
lea 0xe1(%rip),%rdi # 1ef <.L.str.1>
mov $0x4,%esi
mov $0x5,%edx
mov $0x6,%ecx
mov $0x7,%r8d
xor %eax,%eax
callq 190 </PLprintf>
xor %eax,%eax
pop %rbp
retq</pre>
</div>
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