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Integrate Jeffery Yasskin's suggestions with respect to

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traps flowing through memory references, add some text to
better cover phi nodes and externally-visible side effects,
add an example of instructions being control-dependent
on a trap value, and reword some of the existing trap rules.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@102399 91177308-0d34-0410-b5e6-96231b3b80d8
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Dan Gohman 2010-04-26 23:36:52 +00:00
parent 5dfa87ecc6
commit ae11c3f83e
1 changed files with 61 additions and 12 deletions
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@ -2332,25 +2332,74 @@ has undefined behavior.</p>
effects has nevertheless detected a condition which results in undefined
behavior.</p>
<p>Any non-void instruction or constant expression other than a non-intrinsic
call, invoke, or phi with a trap operand has trap as its result value.
Any instruction with a trap operand which may have side effects emits
those side effects as if it had an undef operand instead.</p>
<p>Any value other than a non-intrinsic call, invoke, or phi with a trap
operand has trap as its result value. Any instruction with
a trap operand which may have side effects emits those side effects as
if it had an undef operand instead. If the side effects are externally
visible, the behavior is undefined.</p>
<p>Trap values may be stored to memory; a load from memory including any
part of a trap value results in a (full) trap value.</p>
<p>For example:</p>
<!-- FIXME: In the case of multiple threads, this only applies to loads from
the same thread as the store, or loads which are sequenced after the
store by synchronization. -->
<div class="doc_code">
<pre>
%trap = sub nuw i32 0, 1 ; Results in a trap value.
%still_trap = and i32 %trap, 0 ; Whereas (and i32 undef, 0) would return 0.
%trap_yet_again = getelementptr i32* @h, i32 %still_trap
store i32 0, i32* %trap_yet_again ; undefined behavior
volatile store i32 %trap, i32* @g ; External observation; undefined behavior.
%trap2 = load i32* @g ; Returns a trap value, not just undef.
%narrowaddr = bitcast i32* @g to i16*
%wideaddr = bitcast i32* @g to i64*
%trap3 = load 16* %narrowaddr ; Returns a trap value
%trap4 = load i64* %widaddr ; Returns a trap value, not partial trap.
</pre>
</div>
<p>If a <a href="#i_br"><tt>br</tt></a> or
<a href="#i_switch"><tt>switch</tt></a> instruction has a trap value
operand, all non-phi non-void instructions which control-depend on it
have trap as their result value. If any instruction which
control-depends on the <tt>br</tt> or <tt>switch</tt> invokes externally
visible side effects, the behavior of the program is undefined.</p>
have trap as their result value. A <a href="#i_phi"><tt>phi</tt></a>
node with an incoming value associated with a control edge which is
control-dependent on it has trap as its result value when control is
transferred from that block. If any instruction which control-depends
on the <tt>br</tt> or <tt>switch</tt> invokes externally visible side
effects, the behavior of the program is undefined. For example:</p>
<!-- FIXME: What about exceptions thrown from control-dependent instrs? -->
<p>For example, an <a href="#i_and"><tt>and</tt></a> of a trap value with
zero still has a trap value result. Using that value as an index in a
<a href="#i_getelementptr"><tt>getelementptr</tt></a> yields a trap
result. Using that result as the address of a
<a href="#i_store"><tt>store</tt></a> produces undefined behavior.</p>
<div class="doc_code">
<pre>
entry:
%trap = sub nuw i32 0, 1 ; Results in a trap value.
%cmp = icmp i32 slt %trap, 0 ; Still trap.
%br i1 %cmp, %true, %end ; Branch to either destination.
true:
volatile store i32 0, i32* @g ; Externally visible side effects
; control-dependent on %cmp.
; Undefined behavior.
br label %end
end:
%p = phi i32 [ 0, %entry ], [ 1, %true ]
; Both edges into this PHI are
; control-dependent on %cmp, so this
; results in a trap value.
volatile store i32 0, i32* @g ; %end is control-equivalent to %entry
; so this is defined (ignoring earlier
; undefined behavior in this example).
</pre>
</div>
<p>There is currently no way of representing a trap constant in the IR; they
only exist when produced by certain instructions, such as an