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[LangRef] Add poison constant

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This patch adds a description about the newly added poison constant to LangRef.

Differential Revision: https://reviews.llvm.org/D92162
This commit is contained in:
Juneyoung Lee 2020-11-26 19:33:42 +09:00
parent 5962dadeb0
commit 7546d005c8
1 changed files with 6 additions and 5 deletions
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docs/LangRef.rst
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@ -3657,13 +3657,13 @@ input IR as well.
Poison Values
-------------
A poison value is a result of an erroneous operation.
In order to facilitate speculative execution, many instructions do not
invoke immediate undefined behavior when provided with illegal operands,
and return a poison value instead.
There is currently no way of representing a poison value in the IR; they
only exist when produced by operations such as :ref:`add <i_add>` with
the ``nsw`` flag.
The string '``poison``' can be used anywhere a constant is expected, and
operations such as :ref:`add <i_add>` with the ``nsw`` flag can produce
a poison value.
Poison value behavior is defined in terms of value *dependence*:
@ -3732,13 +3732,14 @@ Here are some examples:
entry:
%poison = sub nuw i32 0, 1 ; Results in a poison value.
%poison2 = sub i32 poison, 1 ; Also results in a poison value.
%still_poison = and i32 %poison, 0 ; 0, but also poison.
%poison_yet_again = getelementptr i32, i32* @h, i32 %still_poison
store i32 0, i32* %poison_yet_again ; Undefined behavior due to
; store to poison.
store i32 %poison, i32* @g ; Poison value stored to memory.
%poison2 = load i32, i32* @g ; Poison value loaded back from memory.
%poison3 = load i32, i32* @g ; Poison value loaded back from memory.
%narrowaddr = bitcast i32* @g to i16*
%wideaddr = bitcast i32* @g to i64*