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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - The _mm_movemask_epi8 intrinsic will sometimes call pmovmskb and will other times call movmskps"
href="https://bugs.llvm.org/show_bug.cgi?id=52567">52567</a>
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<th>Summary</th>
<td>The _mm_movemask_epi8 intrinsic will sometimes call pmovmskb and will other times call movmskps
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>Linux
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: X86
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>chrisburel@gmail.com
</td>
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<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, pengfei.wang@intel.com, spatel+llvm@rotateright.com
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<pre>I've found that calling _mm_movemask_epi8 changes its behavior depending on the
way that it is used in surrounding expressions.
This is reproducible with this code:
1 #include <xmmintrin.h>
2 int main()
3 {
4 const int mask = 0x00FF;
5 const auto recip = _mm_rcp_ps(_mm_set_ps(0.0f, 0.0f, 4.0f, 2.0f));
6 const auto diff = _mm_sub_ps(_mm_set_ps(0.0f, 0.0f, 0.25f, 0.5f),
recip);
7 const auto abs = _mm_and_ps(diff, _mm_set1_epi32(0x7FFFFFFF));
8 const auto compare = _mm_castps_si128(_mm_cmpgt_ps(abs,
_mm_set1_ps(0.001f)));
9 return (_mm_movemask_epi8(compare) & mask) == 0;
10 }
In plain English, this code:
1. Gets the reciprocal of the vector (0.0f, 0.0f, 4.0f, 2.0f)
2. Subtracts (0.0f, 0.0f, 0.25f, 0.5f) from the result of 1
3. Takes the absolute value of the result of 2
4. Returns 0 if the last 2 values of the result of 3 are greater than 0.001f, 1
otherwise
Intel's Intrinsics guide states that the _mm_movemask_epi8 intrinsic function
corresponds to the "pmovmskb r32, xmm" instruction. Thus, it should return
0xff00. However, because the value of mask (declared on line 4) is known at
compile time, the call to _mm_movemask_epi8 on line 9 generates a movmskps
instruction, which returns a different value. Instead of 16 1 bits per true
value 0xff00, we only get 1 true bit per value, 0b1100.
Assembly that is generated for this code is:
rcpps xmm0, xmmword ptr [rip + .LCPI0_0]
movaps xmm1, xmmword ptr [rip + .LCPI0_1] # xmm1 =
[5.0E-1,2.5E-1,0.0E+0,0.0E+0]
subps xmm1, xmm0
andps xmm1, xmmword ptr [rip + .LCPI0_2]
movaps xmm0, xmmword ptr [rip + .LCPI0_3] # xmm0 =
[1.00000005E-3,1.00000005E-3,1.00000005E-3,1.00000005E-3]
cmpltps xmm0, xmm1
movmskps ecx, xmm0 # <--- this is the instruction from
_mm_movemask_epi8
xor eax, eax
test ecx, ecx
sete al
ret
If we do anything to the mask to force the compiler's hand, it will call
pmovmskb instead. The simplest thing to do is to mark the mask variable as
volatile, but moving it to a separate TU also works.
rcpps xmm0, xmmword ptr [rip + .LCPI0_0]
movaps xmm1, xmmword ptr [rip + .LCPI0_1] # xmm1 =
[5.0E-1,2.5E-1,0.0E+0,0.0E+0]
subps xmm1, xmm0
andps xmm1, xmmword ptr [rip + .LCPI0_2]
mov dword ptr [rsp - 4], 255
movaps xmm0, xmmword ptr [rip + .LCPI0_3] # xmm0 =
[1.00000005E-3,1.00000005E-3,1.00000005E-3,1.00000005E-3]
cmpltps xmm0, xmm1
pmovmskb ecx, xmm0 # <--- this is the instruction from
_mm_movemask_epi8
xor eax, eax
and ecx, dword ptr [rsp - 4]
sete al
ret
The result is that the return value of _mm_movemask_epi8 is unpredictable.
The code can be seen here: <a href="https://godbolt.org/z/3Yc734rnj">https://godbolt.org/z/3Yc734rnj</a>
The top code is the current behavior, the bottom code only differs in that mask
is volatile.
I used git bisect and found that this behavior changed in git commit
0741b75ad543d108759c0658fedb5fdfcf064487.</pre>
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