describe undef semantics in some more detail.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@81167 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2009-09-07 22:52:39 +00:00
parent ef17e24aee
commit 48a109c36c

View File

@ -2016,13 +2016,109 @@ Classifications</a> </div>
<div class="doc_subsection"><a name="undefvalues">Undefined Values</a></div> <div class="doc_subsection"><a name="undefvalues">Undefined Values</a></div>
<div class="doc_text"> <div class="doc_text">
<p>The string '<tt>undef</tt>' is recognized as a type-less constant that has no <p>The string '<tt>undef</tt>' can be used anywhere a constant is expected, and
specific value. Undefined values may be of any type and be used anywhere a indicates that the user of the value may recieve an unspecified bit-pattern.
constant is permitted.</p> Undefined values may be of any type (other than label or void) and be used
anywhere a constant is permitted.</p>
<p>Undefined values indicate to the compiler that the program is well defined no <p>Undefined values are useful, because it indicates to the compiler that the
matter what value is used, giving the compiler more freedom to optimize.</p> program is well defined no matter what value is used. This gives the
compiler more freedom to optimize. Here are some examples of (potentially
surprising) transformations that are valid (in pseudo IR):</p>
<div class="doc_code">
<pre>
%A = add %X, undef
%B = sub %X, undef
%C = xor %X, undef
Safe:
%A = undef
%B = undef
%C = undef
</pre>
</div>
<p>This is safe because all of the output bits are affected by the undef bits.
Any output bit can have a zero or one depending on the input bits.</p>
<div class="doc_code">
<pre>
%A = or %X, undef
%B = and %X, undef
Safe:
%A = -1
%B = 0
Unsafe:
%A = undef
%B = undef
</pre>
</div>
<p>These logical operations have bits that are not always affected by the input.
For example, if "%X" has a zero bit, then the output of the 'and' operation will
always be a zero, no matter what the corresponding bit from the undef is. As
such, it is unsafe to optimizer or assume that the result of the and is undef.
However, it is safe to assume that all bits of the undef are 0, and optimize the
and to 0. Likewise, it is safe to assume that all the bits of the undef operand
to the or could be set, allowing the or to be folded to -1.</p>
<div class="doc_code">
<pre>
%A = select undef, %X, %Y
%B = select undef, 42, %Y
%C = select %X, %Y, undef
Safe:
%A = %X (or %Y)
%B = 42 (or %Y)
%C = %Y
Unsafe:
%A = undef
%B = undef
%C = undef
</pre>
</div>
<p>This set of examples show that undefined select (and conditional branch)
conditions can go "either way" but they have to come from one of the two
operands. In the %A example, if %X and %Y were both known to have a clear low
bit, then %A would have to have a cleared low bit. However, in the %C example,
the optimizer is allowed to assume that the undef operand could be the same as
%Y, allowing the whole select to be eliminated.</p>
<div class="doc_code">
<pre>
%A = xor undef, undef
%B = undef
%C = xor %B, %B
%D = undef
%E = icmp lt %D, 4
%F = icmp gte %D, 4
Safe:
%A = undef
%B = undef
%C = undef
%D = undef
%E = undef
%F = undef
</pre>
</div>
<p>This example points out that two undef operands are not necessarily the same.
This can be surprising to people (and also matches C semantics) where they
assume that "X^X" is always zero, even if X is undef. This isn't true for a
number of reasons, but the short answer is that an undef "variable" can
arbitrarily change its value over its "live range". This is true because the
"variable" doesn't actually <em>have a live range</em>. Instead, the value is
logically read from arbitrary registers that happen to be around when needed,
so the value is not neccesarily consistent over time. In fact, %A and %C need
to have the same semantics of the core LLVM "replace all uses with" concept
would not hold.</p>
</div> </div>
<!-- ======================================================================= --> <!-- ======================================================================= -->