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diff --git a/docs/LangRef.html b/docs/LangRef.html index 4308d31..596066d 100644 --- a/docs/LangRef.html +++ b/docs/LangRef.html @@ -2118,7 +2118,46 @@ 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 class="doc_code"> +<pre> + %A = fdiv undef, %X + %B = fdiv %X, undef +Safe: + %A = undef +b: unreachable +</pre> +</div> + +<p>These examples show the crucial difference between an <em>undefined +value</em> and <em>undefined behavior</em>. An undefined value (like undef) is +allowed to have an arbitrary bit-pattern. This means that the %A operation +can be constant folded to undef because the undef could be an SNaN, and fdiv is +not (currently) defined on SNaN's. However, in the second example, we can make +a more aggressive assumption: because the undef is allowed to be an arbitrary +value, we are allowed to assume that it could be zero. Since a divide by zero +is has <em>undefined behavior</em>, we are allowed to assume that the operation +does not execute at all. This allows us to delete the divide and all code after +it: since the undefined operation "can't happen", the optimizer can assume that +it occurs in dead code. +</p> + +<div class="doc_code"> +<pre> +a: store undef -> %X +b: store %X -> undef +Safe: +a: <deleted> +b: unreachable +</pre> +</div> + +<p>These examples reiterate the fdiv example: a store "of" an undefined value +can be assumed to not have any effect: we can assume that the value is +overwritten with bits that happen to match what was already there. However, a +store "to" an undefined location could clobber arbitrary memory, therefore, it +has undefined behavior.</p> + </div> <!-- ======================================================================= --> |