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Describe the reasoning for compact unwind in better terms. Thanks to Nick Kledzik for the description.

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@136064 91177308-0d34-0410-b5e6-96231b3b80d8
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Bill Wendling 2011-07-26 07:58:09 +00:00
parent 5d348b4dc4
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@ -1813,12 +1813,14 @@ $ llc -regalloc=pbqp file.bc -o pbqp.s;
<div>
<p>Unwinding out of a function is done virually via DWARF encodings. These
encodings exist in two forms: a Common Information Entry (CIE) and a Frame
Description Entry (FDE). These two tables contain the information necessary
for the unwinder to restore the state of the computer to before the function
was called. However, the tables themselves are rather large. LLVM can use a
"compact unwind" encoding to represent the virtual unwinding.</p>
<p>Throwing an exception requires <em>unwinding</em> out of a function. The
information on how to unwind a given function is traditionally expressed in
DWARF unwind (a.k.a. frame) info. But that format was originally developed
for debuggers to backtrace, and each Frame Description Entry (FDE) requires
~20-30 bytes per function. There is also the cost of mapping from an address
in a function to the corresponding FDE at runtime. An alternative unwind
encoding is called <em>compact unwind</em> and requires just 4-bytes per
function.</p>
<p>The compact unwind encoding is a 32-bit value, which is encoded in an
architecture-specific way. It specifies which registers to restore and from
@ -1834,7 +1836,7 @@ $ llc -regalloc=pbqp file.bc -o pbqp.s;
<p>For X86, there are three modes for the compact unwind encoding:</p>
<ul>
<dl>
<dt><i>Function with a Frame Pointer (<code>EBP</code> or <code>RBP</code>)</i></dt>
<dd><p><code>EBP/RBP</code>-based frame, where <code>EBP/RBP</code> is pushed
onto the stack immediately after the return address,
@ -1845,10 +1847,11 @@ $ llc -regalloc=pbqp file.bc -o pbqp.s;
more into the PC. All non-volatile registers that need to be restored must
have been saved in a small range on the stack that
starts <code>EBP-4</code> to <code>EBP-1020</code> (<code>RBP-8</code>
to <code>RBP-1020</code>). The offset (divided by 4) is encoded in bits
16-23 (mask: <code>0x00FF0000</code>). The registers saved are encoded in
bits 0-14 (mask: <code>0x00007FFF</code>) as five 3-bit entries from the
following table:</p>
to <code>RBP-1020</code>). The offset (divided by 4 in 32-bit mode and 8
in 64-bit mode) is encoded in bits 16-23 (mask: <code>0x00FF0000</code>).
The registers saved are encoded in bits 0-14
(mask: <code>0x00007FFF</code>) as five 3-bit entries from the following
table:</p>
<table border="1" cellspacing="0">
<tr>
<th>Compact Number</th>
@ -1895,13 +1898,14 @@ $ llc -regalloc=pbqp file.bc -o pbqp.s;
to the <code>ESP/RSP</code>. Then the return is done by popping the stack
into the PC. All non-volatile registers that need to be restored must have
been saved on the stack immediately after the return address. The stack
size (divided by 4) is encoded in bits 16-23
(mask: <code>0x00FF0000</code>). There is a maximum stack size of 1024
bytes. The number of registers saved is encoded in bits 9-12
(mask: <code>0x00001C00</code>). Bits 0-9 (mask:
<code>0x000003FF</code>) contain which registers were saved and their
order. (See the <code>encodeCompactUnwindRegistersWithoutFrame()</code>
function in <code>lib/Target/X86FrameLowering.cpp</code> for the encoding
size (divided by 4 in 32-bit mode and 8 in 64-bit mode) is encoded in bits
16-23 (mask: <code>0x00FF0000</code>). There is a maximum stack size of
1024 bytes in 32-bit mode and 2048 in 64-bit mode. The number of registers
saved is encoded in bits 9-12 (mask: <code>0x00001C00</code>). Bits 0-9
(mask: <code>0x000003FF</code>) contain which registers were saved and
their order. (See
the <code>encodeCompactUnwindRegistersWithoutFrame()</code> function
in <code>lib/Target/X86FrameLowering.cpp</code> for the encoding
algorithm.)</p></dd>
<dt><i>Frameless with a Large Constant Stack Size (<code>EBP</code>
@ -1912,7 +1916,7 @@ $ llc -regalloc=pbqp file.bc -o pbqp.s;
$nnnnnn, %esp</code>" in its prolog. The compact encoding contains the
offset to the <code>$nnnnnn</code> value in the function in bits 9-12
(mask: <code>0x00001C00</code>).</p></dd>
</ul>
</dl>
</div>