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13.22 Structure Sharing Assumptions ¶

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 expressionobject of a certain kind appears in more than one place in thecontaining structure.

These assumptions refer to a single function; except for the RTLobjects 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 singlereg object to represent it,and therefore only a single machine mode.
For any symbolic label, there is only onesymbol_ref objectreferring to it.
Allconst_int expressions with equal values are shared.
Allconst_poly_int expressions with equal modes and valuesare shared.
There is only onepc expression.
There is only oneconst_double expression with value 0 foreach floating point mode. Likewise for values 1 and 2.
There is only oneconst_vector expression with value 0 foreach vector mode, be it an integer or a double constant vector.
Nolabel_ref orscratch appears in more than one place inthe RTL structure; in other words, it is safe to do a tree-walk of allthe insns in the function and assume that each time alabel_reforscratch is seen it is distinct from all others that are seen.
Only onemem object is normally created for each staticvariable or stack slot, so these objects are frequently shared in allthe places they appear. However, separate but equal objects for thesevariables are occasionally made.
When a singleasm statement has multiple output operands, adistinctasm_operands expression is made for each output operand.However, these all share the vector which contains the sequence of inputoperands. This sharing is used later on to test whether twoasm_operands expressions come from the same statement, so alloptimizations must carefully preserve the sharing if they copy thevector at all.
No RTL object appears in more than one place in the RTL structureexcept as described above. Many passes of the compiler rely on thisby assuming that they can modify RTL objects in place without unwantedside-effects on other insns.
During initial RTL generation, shared structure is freely introduced.After all the RTL for a function has been generated, all sharedstructure is copied byunshare_all_rtl inemit-rtl.cc,after which the above rules are guaranteed to be followed.
During the combiner pass, shared structure within an insn can existtemporarily. However, the shared structure is copied before thecombiner is finished with the insn. This is done by callingcopy_rtx_if_shared, which is a subroutine ofunshare_all_rtl.