This came up when investigating PR62151. In that PR combine messes up a
four-insn combination. It should really have done the combination of the
first three insns in that. The second of those instructions sets a register;
the third clobbers the same. Substituting the source of the set into the
clobber usually results in invalid RTL so that the combination will not be
valid; also, it confuses the undobuf machinery. can_combine_p rejects
the combination for this reason.
But we can simply make subst not substitute into clobbers of registers.
Any unnecessary clobbers will be removed later anyway.
With this patch, the three-insn combination in PR62151 is successful. It
does likely not really solve the problem there though, it just hides it.
Bootstrapped and regression checked on powerpc64-linux, options
-m64,-m32,-m32/-mpowerpc64.
Is this okay for mainline?
Segher
2014-09-03 Segher Boessenkool <segher@kernel.crashing.org>
PR rtl-optimization/62151
* combine.c (can_combine_p): Allow the destination register of INSN
to be clobbered in I3.
(subst): Do not substitute into clobbers of registers.