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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [ARM] Machine Outliner issue with LDRD/STRD in Thumb mode"
href="https://bugs.llvm.org/show_bug.cgi?id=50481">50481</a>
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<th>Summary</th>
<td>[ARM] Machine Outliner issue with LDRD/STRD in Thumb mode
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<th>Product</th>
<td>libraries
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<th>Version</th>
<td>12.0
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<th>Hardware</th>
<td>PC
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
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<th>Severity</th>
<td>release blocker
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>Backend: ARM
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>yvan.roux@linaro.org
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, smithp352@googlemail.com, Ties.Stuij@arm.com
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<div>
<pre>This issue was reported and described by rust-lang community here:
<a href="https://github.com/rust-lang/rust/issues/85351">https://github.com/rust-lang/rust/issues/85351</a>
The problem occurs when the machine outliner extracts a chunck of code which
contains a call and loads or stores a pair of registers from or into the
stack, such as:
bl foo
strd r0, r1, [sp, #0]
strd r2, r3, [sp, #8]
LR needs to be saved into the stack when jumping in such an outlined
function and stack's offsets should be changed accordingly, thus the
function should look like:
<OUTLINED_FUNCTION_0>:
str.w lr, [sp, #-8]!
bl foo
strd r0, r1, [sp, #8]
strd r2, r3, [sp, #16]
ldr.w lr, [sp], #8
bx lr
But in the code generated by llvm-12 for Thumb2 targets, the stack offsets
are not patched. In fact, these offsets are patched in the MIR by the
Machine Outliner, but it assumes that immediates encoded in
AddrModeT2_i8s4 instructions are scaled like it is the case for the other
addressing modes.
I'll submit a small patch to fix the outliner which will change the scale
and bit number in that part, but maybe it'd be nice to do a bigger change
to make the encoding more consistent (there are already some fixme notes
about that in the code)</pre>
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