http://llvm.org/bugs/show_bug.cgi?id=20895
Bug ID: 20895
Summary: documentation bug: select should not have a value
dependence on the not-selected operand
Product: new-bugs
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: new bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
This PR is in reference to this discussion:
http://lists.cs.uiuc.edu/pipermail/llvmdev/2014-September/076592.html
Basically, the LLVM IR doc implies that "select" has a value dependence on its
operands, which makes it legal to optimize a select into undef if its
not-selected operand is poisoned. This doesn't seem to make too much sense and
also it appears to be at odds with the current LLVM implementation.
My suggested fix to the LLVM Language Reference Manual is to put this text in
the value dependence part of the poison value explanation:
* Values other than phi nodes and select instructions depend on their operands.
* Phi nodes depend on the operand corresponding to their dynamic predecessor
basic block.
* Select instructions depend on their selected operand.
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