The documentation of REG_RETVAL and REG_LIBCALL notes is incomplete. The following additional information should be added to doc/rtl.texi:
1. LIBCALL and RETVAL notes always appear in pairs. 2. Operand 0 the libcall reg note always points to the other note of the pair. 3. Nested pairs of libcall notes are not allowed. Nested libcalls are not allowed, but there is one pass that is prepared to handle them anyway, and that is the local const/copy propagation pass in gcse.c. A patch for that is attached to Bug 33029 (http://gcc.gnu.org/bugzilla/attachment.cgi?id=14064). A patch for the documentation is pasted below: Index: doc/rtl.texi =================================================================== --- doc/rtl.texi (revision 128277) +++ doc/rtl.texi (working copy) @@ -3640,13 +3640,16 @@ library call), and @var{op} is the first library call, the first insn that was generated to set up the arguments for the library call). -Loop optimization uses this note to treat such a sequence as a single -operation for code motion purposes and flow analysis uses this note to -delete such sequences whose results are dead. +The RTL dead code elimination pass uses this note to delete such +sequences whose results are dead. A @code{REG_EQUAL} note will also usually be attached to this insn to provide the expression being computed by the sequence. +A @code{REG_RETVAL} note always appears in pair with a @code{REG_LIBCALL} +note. The first operand of the @code{REG_RETVAL} note is a pointer to +the @code{REG_LIBCALL} note of the pair. + These notes will be deleted after reload, since they are no longer accurate or useful. @@ -3655,6 +3658,10 @@ accurate or useful. This is the inverse of @code{REG_RETVAL}: it is placed on the first insn of a multi-insn sequence, and it points to the last one. +A @code{REG_LIBCALL} note always appears in pair with a @code{REG_LIBCALL} +note. The first operand of the @code{REG_LIBCALL} note is a pointer to +the @code{REG_RETVAL} note of the pair. + These notes are deleted after reload, since they are no longer useful or accurate. -- Summary: Incomplete documentation of REG_RETVAL and REG_LIBCALL notes Product: gcc Version: 4.3.0 Status: UNCONFIRMED Keywords: documentation Severity: normal Priority: P3 Component: rtl-optimization AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: steven at gcc dot gnu dot org BugsThisDependsOn: 33029 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33356
