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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 - GVN-Hoist hoists a load over a predicate"
href="https://bugs.llvm.org/show_bug.cgi?id=32821">32821</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>GVN-Hoist hoists a load over a predicate
</td>
</tr>
<tr>
<th>Product</th>
<td>new-bugs
</td>
</tr>
<tr>
<th>Version</th>
<td>unspecified
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>All
</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>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>chandlerc@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Consider the following test case:
% cat bug.cc
class base
{
public:
base() {}
virtual ~base() {}
virtual void destroy() = 0;
virtual void destroy_deallocate() = 0;
};
class bar
{
public:
bar() : _f(0) {}
~bar()
{
if ((void *)_f == &_buf)
_f->destroy();
else if (_f)
_f->destroy_deallocate();
}
void* _buf;
base* _f;
};
extern "C" {
unsigned sleep (unsigned int __seconds);
}
extern void foo(bar x);
void baz()
{
bar x;
foo(x);
while (1) sleep(10);
}
% clang --target=x86_64-unknown-unknown -S -o - bug.cc -O2 -fno-exceptions
-fno-rtti -mllvm -enable-gvn-hoist=false
.text
.file "bug.cc"
.globl _Z3bazv
.p2align 4, 0x90
.type _Z3bazv,@function
_Z3bazv: # @_Z3bazv
.cfi_startproc
# BB#0: # %entry
pushq %rbp
.Lcfi0:
.cfi_def_cfa_offset 16
.Lcfi1:
.cfi_offset %rbp, -16
movq %rsp, %rbp
.Lcfi2:
.cfi_def_cfa_register %rbp
pushq %rbx
subq $24, %rsp
.Lcfi3:
.cfi_offset %rbx, -24
movq $0, -16(%rbp)
leaq -24(%rbp), %rbx
movq %rbx, %rdi
callq _Z3foo3bar
movq -16(%rbp), %rdi
cmpq %rbx, %rdi
je .LBB0_1
# BB#2: # %if.else.i
testq %rdi, %rdi # NULL TEST
je .LBB0_4
# BB#3: # %if.then4.i
movq (%rdi), %rax # LOAD
callq *24(%rax)
jmp .LBB0_4
.LBB0_1: # %if.then.i
movq (%rdi), %rax
callq *16(%rax)
.p2align 4, 0x90
.LBB0_4: # %while.cond
# =>This Inner Loop Header: Depth=1
movl $10, %edi
callq sleep
jmp .LBB0_4
.Lfunc_end0:
.size _Z3bazv, .Lfunc_end0-_Z3bazv
.cfi_endproc
.ident "clang version 5.0.0 (trunk 301138) (llvm/trunk 301148)"
.section ".note.GNU-stack","",@progbits
versus:
% clang --target=x86_64-unknown-unknown -S -o - bug.cc -O2 -fno-exceptions
-fno-rtti -mllvm -enable-gvn-hoist=true
.text
.file "bug.cc"
.globl _Z3bazv
.p2align 4, 0x90
.type _Z3bazv,@function
_Z3bazv: # @_Z3bazv
.cfi_startproc
# BB#0: # %entry
pushq %rbp
.Lcfi0:
.cfi_def_cfa_offset 16
.Lcfi1:
.cfi_offset %rbp, -16
movq %rsp, %rbp
.Lcfi2:
.cfi_def_cfa_register %rbp
pushq %rbx
subq $24, %rsp
.Lcfi3:
.cfi_offset %rbx, -24
movq $0, -16(%rbp)
leaq -24(%rbp), %rbx
movq %rbx, %rdi
callq _Z3foo3bar
movq -16(%rbp), %rdi
cmpq %rbx, %rdi
movq (%rdi), %rax # CRASH
je .LBB0_1
# BB#2: # %if.else.i
testq %rdi, %rdi # NULL TEST
je .LBB0_4
# BB#3: # %if.then4.i
callq *24(%rax)
jmp .LBB0_4
.LBB0_1: # %if.then.i
callq *16(%rax)
.p2align 4, 0x90
.LBB0_4: # %while.cond
# =>This Inner Loop Header: Depth=1
movl $10, %edi
callq sleep
jmp .LBB0_4
.Lfunc_end0:
.size _Z3bazv, .Lfunc_end0-_Z3bazv
.cfi_endproc
.ident "clang version 5.0.0 (trunk 301138) (llvm/trunk 301148)"
.section ".note.GNU-stack","",@progbits
See the lines marked "NULL TEST", "LOAD", and "CRASH". GVN Hoist is lifting the
load through %rdi in this example across the null test.
You can also see this in the IR, nothing x86 specific here.</pre>
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