[Bug rtl-optimization/79048] New: Unnecessary reload for flags setting insn when operands die

[ubizjak at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79048

            Bug ID: 79048
           Summary: Unnecessary reload for flags setting insn when
                    operands die
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: ubizjak at gmail dot com
  Target Milestone: ---

Following testcase:

extern void g();
void f1(char a,char b){ if(a|b)g(); }

compiles to (-O2):

f1:
        movl    %esi, %eax
        orb     %dil, %al
        jne     .L4

Note that reload fails to figure out that both registers die in (insn 10):

(insn 10 9 11 2 (parallel [
            (set (reg:CCZ 17 flags)
                (compare:CCZ (ior:QI (subreg:QI (reg:SI 91 [ b ]) 0)
                        (subreg:QI (reg:SI 89 [ a ]) 0))
                    (const_int 0 [0])))
            (clobber (scratch:QI))
        ]) "pr79045.c":2 437 {*iorqi_3}
     (expr_list:REG_DEAD (reg:SI 91 [ b ])
        (expr_list:REG_DEAD (reg:SI 89 [ a ])
            (nil))))

RA could allocate %esi (or even %edi, iorqi_3 has commutative operands)
as a matched output scratch, but instead generates an unnecessary reload with
additional temporary register, bloating the code and increasing register
pressure:

(insn 21 9 10 2 (set (reg:QI 0 ax [93])
        (reg:QI 4 si [orig:91 b ] [91])) "pr79045.c":2 84 {*movqi_internal}
     (nil))
(insn 10 21 11 2 (parallel [
            (set (reg:CCZ 17 flags)
                (compare:CCZ (ior:QI (reg:QI 0 ax [93])
                        (reg:QI 5 di [orig:89 a ] [89]))
                    (const_int 0 [0])))
            (clobber (reg:QI 0 ax [93]))
        ]) "pr79045.c":2 437 {*iorqi_3}
     (nil))

Ideally, RA should use dead input reg to allocate as matched scratch, possibly
also exploiting "%" operand modifier.