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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Wrong CFI metadata for registers spilled in prolog"
href="https://bugs.llvm.org/show_bug.cgi?id=49663">49663</a>
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<th>Summary</th>
<td>Wrong CFI metadata for registers spilled in prolog
</td>
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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>Windows NT
</td>
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
</td>
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<th>Priority</th>
<td>P
</td>
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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>simonas+llvm.org@kazlauskas.me
</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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<p>
<div>
<pre>Consider the following snippet (<a href="https://llvm.godbolt.org/z/cTr35q">https://llvm.godbolt.org/z/cTr35q</a>):
target triple = "i686"
define void @no_availble_registers_many_args(i32 %a, i32 %b, i32 %c, i32
%d) "no_caller_saved_registers" {
%all = alloca i32, i64 18000
%ab = add i32 %a, %b
%cd = add i32 %c, %d
%abcd = add i32 %ab, %cd
%b0 = getelementptr inbounds i32, i32* %all, i64 98
store volatile i32 %abcd, i32* %b0
ret void
}
This produces the following assembly in the prologue:
.cfi_startproc
pushl %ecx
.cfi_def_cfa_offset 8
pushl %eax
.cfi_def_cfa_offset 12
subl $72000, %esp # imm = 0x11940
.cfi_def_cfa_offset 72012
.cfi_offset %eax, -12
.cfi_offset %ecx, -8
Here we set the `.cfi_offset` for the spilled registers (eax, ecx) too late and
to the wrong offsets on the stack.
These `.cfi` directives would've been correct if they were emitted immediately
after the `push` instruction, but with the huge allocation on the stack, we
actially `.def_cfa_offset` first, thus changing the offsets entirely, and
making these directives incorrect.</pre>
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