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Diffstat (limited to 'gcc/doc/tree-ssa.texi')
| -rw-r--r-- | gcc/doc/tree-ssa.texi | 20 |
1 files changed, 20 insertions, 0 deletions
diff --git a/gcc/doc/tree-ssa.texi b/gcc/doc/tree-ssa.texi index e2a657a3a13..359f07864f8 100644 --- a/gcc/doc/tree-ssa.texi +++ b/gcc/doc/tree-ssa.texi @@ -1384,6 +1384,26 @@ There are several @code{TODO} flags that control the behavior of renaming are processed@. @end itemize +@subsection Preserving the virtual SSA form +@cindex preserving virtual SSA form + +The virtual SSA form is harder to preserve than the non-virtual SSA form +mainly because the set of virtual operands for a statement may change at +what some would consider unexpected times. In general, any time you +have modified a statement that has virtual operands, you should verify +whether the list of virtual operands has changed, and if so, mark the +newly exposed symbols by callnig @code{mark_new_vars_to_rename}. + +There is one additional caveat to preserving virtual SSA form. When the +entire set of virtual operands may be eliminated due to better +disambiguation, a bare SMT will be added to the list of virtual +operands, to signify the non-visible aliases that the are still being +referenced. If the set of bare SMT's may change, +@code{TODO_update_smt_usage} should be added to the todo flags. + +With the current pruning code, this can only occur when constants are +propagated into array references that were previously non-constant, or +address expressions are propagated into their uses. @subsection Examining @code{SSA_NAME} nodes @cindex examining SSA_NAMEs |