Re: PATCH: Remove REG_EQUAL from insn where REG_RETVAL is added

[Jeffrey A Law <law at cygnus dot com>]

  In message <200004041235.OAA28933@ignucius.axis.se>you write:
  > The reviewer should know that it is documented (and assumed) in several
  > places in GCC that a REG_EQUAL note on the same insn as a REG_RETVAL
  > indicates "the expression being computed by the sequence".  Those places
  > includes delete_trivially_dead_insns, which as you might guess does bad
  > things when the wrong REG_EQUAL note is on the same insn as a REG_RETVAL
  > note.
Yup.


  > I present the "safe" approach here, but it seems preferable to have the
  > set_unique_reg_note call in both the true and false branches below; there
  > seems to be missing optimization opportunities when it's absent.
Yes.  A few of the optimizers look for the REG_EQUAL notes for libcall
sequences and use them to improve optimization.

  > For usage of the note, does it really matter that the movdi is
  > synthesized?
Yes.  Or more correctly, kind-of.

When the movdi is not synthesized we end the sequence with an insn that
copies a value to itself.  That is the definition of a libcall sequence
that can be optimized by cse, loop, etc.


It just happens to be the case that I was looking at a bug today which I
believe your patch will fix (I won't know for a while).  In the mean time
I bootstrapped the x86 linux port with your patch and ran the testsuite
without regressions and installed your patch.

Thanks,
jeff