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<base href="https://llvm.org/bugs/" />
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [x86] extra cmov in clamp function"
href="https://llvm.org/bugs/show_bug.cgi?id=29002">29002</a>
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<tr>
<th>Summary</th>
<td>[x86] extra cmov in clamp function
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</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>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>spatel+llvm@rotateright.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>void clamp(const float *src, short *dst) {
int tmp = (int)((*src) * 32767.0f);
if (tmp > 32767) tmp = 32767;
if (tmp < -32768) tmp = -32768;
*dst = tmp;
}
Or as IR:
define void @clamp(float* %src, i16* %dst) {
%ld = load float, float* %src, align 4
%mul = fmul float %ld, 3.276700e+04
%conv = fptosi float %mul to i32
%cmp = icmp sgt i32 %conv, 32767
%sel1 = select i1 %cmp, i32 32767, i32 %conv
%cmp2 = icmp slt i32 %sel1, -32768
%sel2 = select i1 %cmp2, i32 -32768, i32 %sel1
%conv6 = trunc i32 %sel2 to i16
store i16 %conv6, i16* %dst, align 2
ret void
}
Somehow, 2 select instructions became 3 cmovs:
$ ./llc -o - clampdown.ll
...
_clamp: ## @clamp
movss (%rdi), %xmm0 ## xmm0 = mem[0],zero,zero,zero
mulss LCPI0_0(%rip), %xmm0
cvttss2si %xmm0, %eax
cmpl $32767, %eax ## imm = 0x7FFF
movl $32767, %ecx ## imm = 0x7FFF
cmovlel %eax, %ecx
movw $32767, %dx ## imm = 0x7FFF
cmovlew %ax, %dx
cmpl $-32768, %ecx ## imm = 0x8000
movw $-32768, %ax ## imm = 0x8000
cmovgew %dx, %ax
movw %ax, (%rsi)
retq
gcc 6.1 does:
foo(float const*, short*):
movss .LC0(%rip), %xmm0
movl $-32768, %edx
mulss (%rdi), %xmm0
cvttss2si %xmm0, %eax
cmpl $-32768, %eax
cmovl %edx, %eax
movl $32767, %edx
cmpl $32767, %eax
cmovg %edx, %eax
movw %ax, (%rsi)
ret</pre>
</div>
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