[Bug bootstrap/86518] New: Strengthen bootstrap comparison by not enabling warnings at stage3

[amonakov at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86518

            Bug ID: 86518
           Summary: Strengthen bootstrap comparison by not enabling
                    warnings at stage3
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: bootstrap
          Assignee: unassigned at gcc dot gnu.org
          Reporter: amonakov at gcc dot gnu.org
  Target Milestone: ---

Currently stage2 and 3 use the same warning options, but that is redundant: if
any warnings are generated, they will be present at stage2 (and stop
bootstrap). By not enabling any warnings for stage3, we would get checking that
warnings do not affect code generation.

Note that simply adding -w at stage3 doesn't work, as it simply suppresses the
warning at print time.

I tried leaving only -Wno-narrowing in warning flags and got many comparison
failures:

Comparing stages 2 and 3
warning: gcc/cc1obj-checksum.o differs
Bootstrap comparison failure!
gcc/calls.o differs
gcc/dwarf2out.o differs
gcc/loop-iv.o differs
gcc/generic-match.o differs
gcc/ipa-inline.o differs
gcc/builtins.o differs
gcc/optabs.o differs
gcc/tree-vrp.o differs
gcc/profile.o differs
gcc/i386.o differs
gcc/cfgexpand.o differs
gcc/simplify-rtx.o differs
gcc/gimple-ssa-sprintf.o differs
gcc/expr.o differs
gcc/print-tree.o differs
gcc/gimple-match.o differs
gcc/godump.o differs
gcc/gimple-ssa-nonnull-compare.o differs
gcc/targhooks.o differs
gcc/tree-ssa-live.o differs
gcc/gimple-ssa-warn-restrict.o differs
gcc/tree-ssa-ccp.o differs
gcc/gimplify.o differs
gcc/tree-cfg.o differs
gcc/tree-pretty-print.o differs
make: *** [compare] Error 1