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Fix a hole in the definition of "dependence" used by trap values. Trap 

values are also transmitted through branches which cause side effects to
be skipped altogether.

llvm-svn: 129404
This commit is contained in:
Dan Gohman 2011-04-12 23:05:59 +00:00
parent dbfde42468
commit c8454ee995
1 changed files with 22 additions and 2 deletions
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24
llvm/docs/LangRef.html
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@ -2371,6 +2371,11 @@ b: unreachable
is always executed when control transfers to one of the successors, and
may not be executed when control is transfered to another.</li>
<li>Additionally, an instruction also <i>control-depends</i> on a terminator
instruction if the set of instructions it otherwise depends on would be
different if the terminator had transfered control to a different
successor.</li>
<li>Dependence is transitive.</li>
</ul>
@ -2413,8 +2418,23 @@ end:
; control-dependent on %cmp, so this
; always results in a trap value.
volatile store i32 0, i32* @g ; %end is control-equivalent to %entry
; so this is defined (ignoring earlier
volatile store i32 0, i32* @g ; This would depend on the store in %true
; if %cmp is true, or the store in %entry
; otherwise, so this is undefined behavior.
%br i1 %cmp, %second_true, %second_end
; The same branch again, but this time the
; true block doesn't have side effects.
second_true:
; No side effects!
br label %end
second_end:
volatile store i32 0, i32* @g ; This time, the instruction always depends
; on the store in %end. Also, it is
; control-equivalent to %end, so this is
; well- defined (again, ignoring earlier
; undefined behavior in this example).
</pre>