RPCS3/llvm-mirror
1
0
Fork 0
mirror of https://github.com/RPCS3/llvm-mirror.git synced 2024-11-26 12:43:36 +01:00
Code Issues Projects Releases Wiki Activity

[LangRef] Add poison constant

Browse Source 
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
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

11
docs/LangRef.rst
View File

@ -3657,13 +3657,13 @@ input IR as well.
Poison Values Poison Values
------------- -------------
A poison value is a result of an erroneous operation.
In order to facilitate speculative execution, many instructions do not In order to facilitate speculative execution, many instructions do not
invoke immediate undefined behavior when provided with illegal operands, invoke immediate undefined behavior when provided with illegal operands,
and return a poison value instead. and return a poison value instead.
The string '``poison``' can be used anywhere a constant is expected, and
There is currently no way of representing a poison value in the IR; they operations such as :ref:`add <i_add>` with the ``nsw`` flag can produce
only exist when produced by operations such as :ref:`add <i_add>` with a poison value.
the ``nsw`` flag.
Poison value behavior is defined in terms of value *dependence*: Poison value behavior is defined in terms of value *dependence*:
@ -3732,13 +3732,14 @@ Here are some examples:
entry: entry:
%poison = sub nuw i32 0, 1 ; Results in a poison value. %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. %still_poison = and i32 %poison, 0 ; 0, but also poison.
%poison_yet_again = getelementptr i32, i32* @h, i32 %still_poison %poison_yet_again = getelementptr i32, i32* @h, i32 %still_poison
store i32 0, i32* %poison_yet_again ; Undefined behavior due to store i32 0, i32* %poison_yet_again ; Undefined behavior due to
; store to poison. ; store to poison.
store i32 %poison, i32* @g ; Poison value stored to memory. 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* %narrowaddr = bitcast i32* @g to i16*
%wideaddr = bitcast i32* @g to i64* %wideaddr = bitcast i32* @g to i64*