Possible regression in DF analysis

[Jeff Law]

On 12/8/22 04:51, Claudiu Zissulescu Ianculescu via Gcc wrote:
> Hi Eric,
> The problem shows in loop-doloop.c when I introduce a loop end pattern
> that replaces the first jump instruction (JUMP_INSN 15) with a pattern
> that clobbers CC reg. However, the DF doesn't look like it works as
> the doloop step cannot find the CC reg alive. Please see
> loop-doloop.c:766. Hence, it introduces the doloop_end patterns, and
> renders the compare instruction (INSN 14) dead code. leading to
But simply inserting an instruction that clobbers CC can't be a valid 
thing to do.  You have to verify that the register isn't live at the 
point where you want to do the insertion.

Or is that the core of the problem -- that life analysis is inaccurate 
or unavailable?

Jeff