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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 --- - Shrink-wrapping breaks frameless indirect compact unwind emission"
href="https://llvm.org/bugs/show_bug.cgi?id=25614">25614</a>
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<th>Summary</th>
<td>Shrink-wrapping breaks frameless indirect compact unwind emission
</td>
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
</tr>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>normal
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>MC
</td>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>cdavis5x@gmail.com
</td>
</tr>
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org
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<th>Classification</th>
<td>Unclassified
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</tr></table>
<p>
<div>
<pre>The compact unwind emitters (at least, the x86 one) currently assume that the
prologue is at the beginning of the function. That way, for a frameless
function with a large stack allocation, it can figure the offset of the stack
allocation amount based solely on the CFI.
Shrink-wrapping breaks this assumption, because now the prologue may no longer
be at the beginning of the function. But, for a frameless function with a large
stack allocation, the compact unwind emitter stores the offset of the stack
allocation amount relative to the prologue, not the beginning of the function.
Thus, when the unwinder reads the unwind information for this function, it will
happily use the offset given in the unwind info... and get a bogus result,
leading to a crash in the unwinder... or worse, no crash and a bad
backtrace/unwind.
This assembly test case demonstrates the problem:
.section __TEXT,__text,regular,pure_instructions
.globl _test2
_test2: ## @test2
.cfi_startproc
## BB#0: ## %prologue
movzbl _guard(%rip), %eax
andl $1, %eax
cmpl $1, %eax
jne LBB0_2
## BB#1:
retq
LBB0_2: ## %.0.entry
subq $3232, %rsp ## imm = 0xca0
Ltmp20:
.cfi_def_cfa_offset 3240
callq _foo
addq $3232, %rsp ## imm = 0xca0
retq
.cfi_endproc
.subsections_via_symbols
Note the code preceding the prologue, checking to see if the function should be
executed. We should produce a compact encoding of 0x03132000, but we wind up
producing 0x03032000 (i.e. stack allocation offset is 3 when it should be 19).
Unfortunately, at this point, the information we need (i.e. the offset of the
prologue) is missing. We certainly won't have this information for raw
assembly.</pre>
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