1 files changed, 34 insertions, 17 deletions
diff --git a/contrib/gcc/doc/rtl.texi b/contrib/gcc/doc/rtl.texi
index 302957c..605aad0 100644
--- a/contrib/gcc/doc/rtl.texi
+++ b/contrib/gcc/doc/rtl.texi
@@ -608,9 +608,22 @@ Stored in the @code{integrated} field and printed as @samp{/i}.
 @cindex @code{concat} and @samp{/u}
 @cindex @code{unchanging}, in @code{reg} and @code{mem}
 @item RTX_UNCHANGING_P (@var{x})
-Nonzero in a @code{reg}, @code{mem}, or @code{concat} if the memory
-is set at most once,
-anywhere.  This does not mean that it is function invariant.
+Nonzero in a @code{reg}, @code{mem}, or @code{concat} if the register or
+memory is set at most once, anywhere.  This does not mean that it is
+function invariant.
+
+GCC uses this flag to determine whether two references conflict.  As
+implemented by @code{true_dependence} in @file{alias.c} for memory
+references, unchanging memory can't conflict with non-unchanging memory;
+a non-unchanging read can conflict with a non-unchanging write; an
+unchanging read can conflict with an unchanging write (since there may
+be a single store to this address to initialize it); and an unchanging
+store can conflict with a non-unchanging read.  This means we must make
+conservative assumptions when choosing the value of this flag for a
+memory reference to an object containing both unchanging and
+non-unchanging fields: we must set the flag when writing to the object
+and clear it when reading from the object.
+
 Stored in the @code{unchanging} field and printed as @samp{/u}.
 
 @findex SCHED_GROUP_P
@@ -687,13 +700,6 @@ Promoted variables are always either sign- or zero-extended to the wider
 mode on every assignment.  Stored in the @code{in_struct} field and
 printed as @samp{/s}.
 
-@findex SYMBOL_REF_FLAG
-@cindex @code{symbol_ref} and @samp{/v}
-@cindex @code{volatil}, in @code{symbol_ref}
-@item SYMBOL_REF_FLAG (@var{x})
-In a @code{symbol_ref}, this is used as a flag for machine-specific purposes.
-Stored in the @code{volatil} field and printed as @samp{/v}.
-
 @findex SYMBOL_REF_USED
 @cindex @code{used}, in @code{symbol_ref}
 @item SYMBOL_REF_USED (@var{x})
@@ -707,6 +713,17 @@ once.  Stored in the @code{used} field.
 @item SYMBOL_REF_WEAK (@var{x})
 In a @code{symbol_ref}, indicates that @var{x} has been declared weak.
 Stored in the @code{integrated} field and printed as @samp{/i}.
+
+@findex SYMBOL_REF_FLAG
+@cindex @code{symbol_ref} and @samp{/v}
+@cindex @code{volatil}, in @code{symbol_ref}
+@item SYMBOL_REF_FLAG (@var{x})
+In a @code{symbol_ref}, this is used as a flag for machine-specific purposes.
+Stored in the @code{volatil} field and printed as @samp{/v}.
+
+Most uses of @code{SYMBOL_REF_FLAG} are historic and may be subsumed
+by @code{SYMBOL_REF_FLAGS}.  Certainly use of @code{SYMBOL_REF_FLAGS}
+is mandatory if the target requires more than one bit of storage.
 @end table
 
 These are the fields to which the above macros refer:
@@ -2195,9 +2212,9 @@ the operands of these.
 @item (set @var{lval} @var{x})
 Represents the action of storing the value of @var{x} into the place
 represented by @var{lval}.  @var{lval} must be an expression
-representing a place that can be stored in: @code{reg} (or @code{subreg}
-or @code{strict_low_part}), @code{mem}, @code{pc}, @code{parallel}, or
-@code{cc0}.
+representing a place that can be stored in: @code{reg} (or @code{subreg},
+@code{strict_low_part} or @code{zero_extract}), @code{mem}, @code{pc},
+@code{parallel}, or @code{cc0}.
 
 If @var{lval} is a @code{reg}, @code{subreg} or @code{mem}, it has a
 machine mode; then @var{x} must be valid for that mode.
@@ -2210,10 +2227,10 @@ rest of the register receives an undefined value.  Likewise, if
 the mode of the register, the rest of the register can be changed in
 an undefined way.
 
-If @var{lval} is a @code{strict_low_part} of a @code{subreg}, then the
-part of the register specified by the machine mode of the
-@code{subreg} is given the value @var{x} and the rest of the register
-is not changed.
+If @var{lval} is a @code{strict_low_part} or @code{zero_extract} 
+of a @code{subreg}, then the part of the register specified by the
+machine mode of the @code{subreg} is given the value @var{x} and
+the rest of the register is not changed.
 
 If @var{lval} is @code{(cc0)}, it has no machine mode, and @var{x} may
 be either a @code{compare} expression or a value that may have any mode.