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(gcc-295.info)Sharing

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Structure Sharing Assumptions
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   The compiler assumes that certain kinds of RTL expressions are
unique; there do not exist two distinct objects representing the same
value.  In other cases, it makes an opposite assumption: that no RTL
expression object of a certain kind appears in more than one place in
the containing structure.

   These assumptions refer to a single function; except for the RTL
objects that describe global variables and external functions, and a
few standard objects such as small integer constants, no RTL objects
are common to two functions.

   * Each pseudo-register has only a single `reg' object to represent
     it, and therefore only a single machine mode.

   * For any symbolic label, there is only one `symbol_ref' object
     referring to it.

   * There is only one `const_int' expression with value 0, only one
     with value 1, and only one with value -1.  Some other integer
     values are also stored uniquely.

   * There is only one `pc' expression.

   * There is only one `cc0' expression.

   * There is only one `const_double' expression with value 0 for each
     floating point mode.  Likewise for values 1 and 2.

   * No `label_ref' or `scratch' appears in more than one place in the
     RTL structure; in other words, it is safe to do a tree-walk of all
     the insns in the function and assume that each time a `label_ref'
     or `scratch' is seen it is distinct from all others that are seen.

   * Only one `mem' object is normally created for each static variable
     or stack slot, so these objects are frequently shared in all the
     places they appear.  However, separate but equal objects for these
     variables are occasionally made.

   * When a single `asm' statement has multiple output operands, a
     distinct `asm_operands' expression is made for each output operand.
     However, these all share the vector which contains the sequence of
     input operands.  This sharing is used later on to test whether two
     `asm_operands' expressions come from the same statement, so all
     optimizations must carefully preserve the sharing if they copy the
     vector at all.

   * No RTL object appears in more than one place in the RTL structure
     except as described above.  Many passes of the compiler rely on
     this by assuming that they can modify RTL objects in place without
     unwanted side-effects on other insns.

   * During initial RTL generation, shared structure is freely
     introduced.  After all the RTL for a function has been generated,
     all shared structure is copied by `unshare_all_rtl' in
     `emit-rtl.c', after which the above rules are guaranteed to be
     followed.

   * During the combiner pass, shared structure within an insn can exist
     temporarily.  However, the shared structure is copied before the
     combiner is finished with the insn.  This is done by calling
     `copy_rtx_if_shared', which is a subroutine of `unshare_all_rtl'.