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<base href="http://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - [ARM64] Miscompile possibly due to incorrect fcsel"
href="http://llvm.org/bugs/show_bug.cgi?id=19700">19700</a>
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</tr>
<tr>
<th>Summary</th>
<td>[ARM64] Miscompile possibly due to incorrect fcsel
</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>Windows NT
</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: AArch64
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>mcrosier@codeaurora.org
</td>
</tr>
<tr>
<th>CC</th>
<td>apazos@codeaurora.org, llvmbugs@cs.uiuc.edu, t.p.northover@gmail.com
</td>
</tr>
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<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Given the below IR the AArch64 backend produces the expected result, while the
ARM64 does not.
linux{169}> more bad.ll
target datalayout = "e-m:e-i64:64-i128:128-n32:64-S128"
target triple = "arm64-none-linux-gnu"
@foo.b = internal unnamed_addr global float -1.000000e+00, align 4
define float @foo(float %a) noinline {
entry:
%cmp = fcmp olt float %a, 1.000000e+00
%selv = select i1 %cmp, float 1.000000e+00, float %a
%0 = load float* @foo.b, align 4
%cmp2 = fcmp une float %0, %selv
%selv7 = select i1 %cmp2, float %selv, float %0
store float %selv7, float* @foo.b, align 4
ret float %selv7
}
define i32 @main() {
entry:
%call = tail call float @foo(float 0.000000e+00)
%not.cmp = fcmp une float %call, 1.000000e+00
%cond = zext i1 %not.cmp to i32
ret i32 %cond
}
linux{170}> clang-arm64-x bad.ll -mcpu=cortex-a53 -O3 -ffast-math -o bad
-static
linux{171}> ./qemu-aarch64 bad
linux{172}> echo $?
1
linux{173}> clang-aarch64-x bad.ll -mcpu=cortex-a53 -O3 -ffast-math -o bad
-static
linux{174}> ./qemu-aarch64 bad
linux{175}> echo $?
0
Looking at the generated assembly,
linux{176}> clang-arm64-x bad.ll -mcpu=cortex-a53 -O3 -ffast-math -o - -S
.text
.file "bad.ll"
.globl foo
.align 2
.type foo,@function
foo: // @foo
// BB#0: // %entry
fmov s1, #1.00000000
fmax s0, s0, s1
adrp x8, foo.b
ldr s1, [x8, :lo12:foo.b]
fcmp s1, s0
fcsel s0, s1, s0, ne <==========
str s0, [x8, :lo12:foo.b]
ret
.Ltmp1:
.size foo, .Ltmp1-foo
The marked fcsel seems incorrect, I believe it should be fcsel s0, s0, s1, ne.</pre>
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