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Clarify the relationship between byval and readonly/

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readnone.  Make clearer that readnone functions do not
dereference pointer arguments.  Do not use the highly
ambiguous "side-effects" in the readonly description
(since such functions can have control flow side-effects,
such as throwing an exception, or looping for ever).


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@57166 91177308-0d34-0410-b5e6-96231b3b80d8
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Duncan Sands 2008-10-06 08:14:18 +00:00
parent d20a00d011
commit edb05dfe48
1 changed files with 19 additions and 14 deletions
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@ -852,13 +852,16 @@ declare signext i8 @returns_signed_char()
to memory, though some targets use it to distinguish between two different
kinds of registers). Use of this attribute is target-specific.</dd>
<dt><tt>byval</tt></dt>
<dt><tt><a name="byval">byval</a></tt></dt>
<dd>This indicates that the pointer parameter should really be passed by
value to the function. The attribute implies that a hidden copy of the
pointee is made between the caller and the callee, so the callee is unable
to modify the value in the callee. This attribute is only valid on LLVM
pointer arguments. It is generally used to pass structs and arrays by
value, but is also valid on pointers to scalars.</dd>
value, but is also valid on pointers to scalars. The copy is considered to
belong to the caller not the callee (for example,
<tt><a href="#readonly">readonly</a></tt> functions should not write to
<tt>byval</tt> parameters).</dd>
<dt><tt>sret</tt></dt>
<dd>This indicates that the pointer parameter specifies the address of a
@ -948,19 +951,21 @@ unwind or exceptional control flow. If the function does unwind, its runtime
behavior is undefined.</dd>
<dt><tt>readnone</tt></dt>
<dd>This attribute indicates that the function computes its result (or its
thrown exception) based strictly on its arguments. It does not read any global
mutable state (e.g. memory, control registers, etc) visible to caller functions.
Furthermore, <tt>readnone</tt> functions never change any state visible to their
caller.
<dt><tt>readonly</tt></dt>
<dd>This function attribute indicates that the function has no side-effects on
the calling function, but that it depends on state (memory state, control
register state, etc) that may be set in the caller. A readonly function always
returns the same value (or throws the same exception) whenever it is called with
a particular set of arguments and global state.</dd>
<dd>This attribute indicates that the function computes its result (or the
exception it throws) based strictly on its arguments, without dereferencing any
pointer arguments or otherwise accessing any mutable state (e.g. memory, control
registers, etc) visible to caller functions. It does not write through any
pointer arguments (including <tt><a href="#byval">byval</a></tt> arguments) and
never changes any state visible to callers.</dd>
<dt><tt><a name="readonly">readonly</a></tt></dt>
<dd>This attribute indicates that the function does not write through any
pointer arguments (including <tt><a href="#byval">byval</a></tt> arguments)
or otherwise modify any state (e.g. memory, control registers, etc) visible to
caller functions. It may dereference pointer arguments and read state that may
be set in the caller. A readonly function always returns the same value (or
throws the same exception) when called with the same set of arguments and global
state.</dd>
</dl>
</div>