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<base href="http://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - IR Generation Introduces an Incorrect Volatile, Preventing Optimization"
href="http://llvm.org/bugs/show_bug.cgi?id=17306">17306</a>
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<th>Summary</th>
<td>IR Generation Introduces an Incorrect Volatile, Preventing Optimization
</td>
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<th>Product</th>
<td>clang
</td>
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<th>Version</th>
<td>unspecified
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Keywords</th>
<td>code-quality
</td>
</tr>
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<th>Severity</th>
<td>normal
</td>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>LLVM Codegen
</td>
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<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>whunt@google.com
</td>
</tr>
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<th>CC</th>
<td>llvmbugs@cs.uiuc.edu
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>When implementing some intrinsics I have come across an IR-gen bug that is
causing clang to produce very bad code around. After looking at it for a bit,
I've narrowed it down to a load that's incorrectly marked as volatile. If that
the volatile is removed by hand and the result compiled, the optimizer
generates the correct code.
//------------------------------------------------------------------------------
// C++ code
//------------------------------------------------------------------------------
#ifdef __clang__
static __inline__ long __attribute__((__always_inline__, __nodebug__))
_InterlockedExchangeAdd(volatile long* _Addend, long _Value) {
return __atomic_add_fetch(_Addend, _Value, 0) - _Value;
}
#else
#include <intrin.h>
#endif
__declspec(noinline) long test(volatile long* p, long a) {
return _InterlockedExchangeAdd(p, a);
}
extern "C" void printf(const char*, ...);
int main() {
volatile long i = 1;
long a = test(&i, 0xf);
printf("%d, %d\n", i, a);
}
//------------------------------------------------------------------------------
// LLVM IR code
//------------------------------------------------------------------------------
define i32 @"\01?test@@YAJPCJJ@Z"(i32* %p, i32 %a) #0 {
entry:
%.atomicdst.i = alloca i32, align 4
%.atomicdst.i.0.cast = bitcast i32* %.atomicdst.i to i8*
call void @llvm.lifetime.start(i64 4, i8* %.atomicdst.i.0.cast)
%0 = atomicrmw volatile add i32* %p, i32 %a monotonic
%1 = add i32 %0, %a
store i32 %1, i32* %.atomicdst.i, align 4
%.atomicdst.i.0.load = load volatile i32* %.atomicdst.i, align 4
%sub.i = sub nsw i32 %.atomicdst.i.0.load, %a
call void @llvm.lifetime.end(i64 4, i8* %.atomicdst.i.0.cast)
ret i32 %sub.i
}
//------------------------------------------------------------------------------
// Bug!! From IR code:
//------------------------------------------------------------------------------
%.atomicdst.i.0.load = load volatile i32* %.atomicdst.i, align 4
Should *not* be marked volatile.
//------------------------------------------------------------------------------
// Incorrect ASM (current)
//------------------------------------------------------------------------------
00091002 E5 50 in eax,50h
00091004 8B 45 08 mov eax,dword ptr [ebp+8]
00091007 8B 4D 0C mov ecx,dword ptr [ebp+0Ch]
0009100A 89 CA mov edx,ecx
0009100C F0 0F C1 10 lock xadd dword ptr [eax],edx
00091010 01 CA add edx,ecx
00091012 89 55 FC mov dword ptr [ebp-4],edx
00091015 8B 45 FC mov eax,dword ptr [ebp-4]
00091018 29 C8 sub eax,ecx
0009101A 83 C4 04 add esp,4
0009101D 5D pop ebp
0009101E C3 ret
//------------------------------------------------------------------------------
// Correct ASM (correctly generated if volatile is removed)
//------------------------------------------------------------------------------
009F1005 F0 0F C1 01 lock xadd dword ptr [ecx],eax
012A1009 C3 ret</pre>
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