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Clarify local SSA form.

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Rename 'local' to 'stack_slot'.
This commit is contained in:
Jakob Stoklund Olesen
2016-01-22 12:13:38 -08:00
parent 189fc7dfb7
commit 74eb6ce901
2 changed files with 42 additions and 10 deletions
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42
docs/langref.rst
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@@ -11,7 +11,8 @@ a *text format* which is used for test cases and debug output. Files containing
Cretonne textual IL have the ``.cton`` filename extension.
This reference uses the text format to describe IL semantics but glosses over
the details of the lexical and syntactic structure of the test format.
the finer details of the lexical and syntactic structure of the format.
Overall structure
=================
@@ -41,8 +42,8 @@ declares a single local variable, ``ss1``.
After the preample follows the :term:`function body` which consists of
:term:`extended basic block`\s, one of which is marked as the :term:`entry
block`. Every EBB ends with a :term:`terminator instruction`, and execution
can never fall through to the next EBB without an explicit branch.
block`. Every EBB ends with a :term:`terminator instruction`, so execution can
never fall through to the next EBB without an explicit branch.
Static single assignment form
-----------------------------
@@ -65,6 +66,20 @@ SSA values: In the entry block, ``v4`` is the initial value. In the loop block
variable during each iteration. Finally, ``v12`` is computed as the induction
variable value for the next iteration.
It can be difficult to generate correct SSA form if the program being converted
into Cretonne IL contains multiple assignments to the same variables. Such
variables can be presented to Cretonne as :term:`stack slot`\s instead. Stack
slots are accessed with the :inst:`stack_store` and :inst:`stack_load`
instructions which behave more like variable accesses in a typical programming
language. Cretonne can perform the necessary dataflow analysis to convert stack
slots to SSA form.
If all values are only used in the same EBB where they are defined, the
function is said to be in :term:`local SSA form`. It is much faster for
Cretonne to verify the correctness of a function in local SSA form since no
complicated control flow analysis is required.
Value types
===========
@@ -180,7 +195,7 @@ Control flow
.. inst:: brz x, EBB(args...)
Branch when zero.
If ``x`` is a :type:`bool` value, take the branch when ``x`` is false. If
``x`` is an integer value, take the branch when ``x = 0``.
@@ -191,7 +206,7 @@ Control flow
.. inst:: brnz x, EBB(args...)
Branch when non-zero.
If ``x`` is a :type:`bool` value, take the branch when ``x`` is true. If
``x`` is an integer value, take the branch when ``x != 0``.
@@ -776,3 +791,20 @@ Glossary
Cretonne function must have exactly one entry block. The types of the
entry block arguments must match the types of arguments in the function
signature.
stack slot
A fixed size memory allocation in the current function's activation
frame. Also called a local variable.
local SSA form
A restricted version of SSA form where all values are defined and used
in the same EBB. A function is in local SSA form iff it is in SSA form
and:
- No branches pass arguments to their target EBB.
- Only the entry EBB may have arguments.
This also implies that there are no branches to the entry EBB.
Local SSA form is easy to generate and fast to verify. It passes data
between EBBs by using stack slots.