perf parse-events: Disable a subset of flex warnings

Rather than disable all warnings with -w, disable specific warnings.

Predicate enabling the warnings on more recent flex versions.

Tested with GCC 9.3.0 and clang 9.0.1.

Committer notes:

The full set of compilers, gcc and clang that this will be tested on
will be on the signed tag when this change goes upstream.

Added -Wno-misleading-indentation to the flex_flags to overcome this on
opensuse tumbleweed when building with clang:

    CC       /tmp/build/perf/util/parse-events-flex.o
    CC       /tmp/build/perf/util/pmu.o
  /tmp/build/perf/util/parse-events-flex.c:5038:13: error: misleading indentation; statement is not part of the previous 'if' [-Werror,-Wmisleading-indentation]
              if ( ! yyg->yy_state_buf )
              ^
  /tmp/build/perf/util/parse-events-flex.c:5036:9: note: previous statement is here
          if ( ! yyg->yy_state_buf )
          ^

And we need to use this to redirect stderr to stdin and then grep in a
way that is acceptable for BusyBox shell:

  2>&1 |

Previously I was using:

  |&

Which seems to be bash specific.

Added -Wno-sign-compare to overcome this on systems such as centos:7:

    CC       /tmp/build/perf/util/parse-events-flex.o
    CC       /tmp/build/perf/util/pmu.o
    CC       /tmp/build/perf/util/pmu-flex.o
  util/parse-events.l: In function 'parse_events_lex':
  /tmp/build/perf/util/parse-events-flex.c:193:36: error: comparison between signed and unsigned integer expressions [-Werror=sign-compare]
                   for ( yyl = n; yyl < yyleng; ++yyl )\
                                      ^
  /tmp/build/perf/util/parse-events-flex.c:204:9: note: in expansion of macro 'YY_LESS_LINENO'

Added -Wno-unused-parameter to overcome this in systems such as
centos:7:

    CC       /tmp/build/perf/util/parse-events-flex.o
    CC       /tmp/build/perf/util/pmu.o
  /tmp/build/perf/util/parse-events-flex.c: In function 'yy_fatal_error':
  /tmp/build/perf/util/parse-events-flex.c:6265:58: error: unused parameter 'yyscanner' [-Werror=unused-parameter]
   static void yy_fatal_error (yyconst char* msg , yyscan_t yyscanner)
                                                            ^
Added -Wno-missing-declarations to build in systems such as centos:6:

  /tmp/build/perf/util/parse-events-flex.c:6313: error: no previous prototype for 'parse_events_get_column'
  /tmp/build/perf/util/parse-events-flex.c:6389: error: no previous prototype for 'parse_events_set_column'

And -Wno-missing-prototypes to cover older compilers:

  -Wmissing-prototypes (C only)
  Warn if a global function is defined without a previous prototype declaration. This warning is issued even if the definition itself provides a prototype. The aim is to detect global functions that fail to be declared in header files.
  -Wmissing-declarations (C only)
  Warn if a global function is defined without a previous declaration. Do so even if the definition itself provides a prototype. Use this option to detect global functions that are not declared in header files.

Older C compilers lack -Wno-misleading-indentation, check if it is
available before using it.

Also needed to check just the first two levels of the flex version, as
the patch was assuming that all versions were of the form x.y.z, and
there are several cases where it is just x.y, breaking the build.

Signed-off-by: Ian Rogers <irogers@google.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jin Yao <yao.jin@linux.intel.com>
Cc: John Garry <john.garry@huawei.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20200619043356.90024-8-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/Build

@@ -221,9 +221,19 @@ $(OUTPUT)util/pmu-bison.c $(OUTPUT)util/pmu-bison.h: util/pmu.y
 	$(Q)$(call echo-cmd,bison)$(BISON) -v $< -d $(PARSER_DEBUG_BISON) \
 		-o $(OUTPUT)util/pmu-bison.c -p perf_pmu_
 
-CFLAGS_parse-events-flex.o  += -w
-CFLAGS_pmu-flex.o           += -w
-CFLAGS_expr-flex.o          += -w
+FLEX_GE_26 := $(shell expr $(shell $(FLEX) --version | sed -e  's/flex \([0-9]\+\).\([0-9]\+\)/\1\2/g') \>\= 26)
+ifeq ($(FLEX_GE_26),1)
+  flex_flags := -Wno-switch-enum -Wno-switch-default -Wno-unused-function -Wno-redundant-decls -Wno-sign-compare -Wno-unused-parameter -Wno-missing-prototypes -Wno-missing-declarations
+  CC_HASNT_MISLEADING_INDENTATION := $(shell echo "int main(void) { return 0 }" | $(CC) -Werror -Wno-misleading-indentation -o /dev/null -xc - 2>&1 | grep -q -- -Wno-misleading-indentation ; echo $$?)
+  ifeq ($(CC_HASNT_MISLEADING_INDENTATION), 1)
+    flex_flags += -Wno-misleading-indentation
+  endif
+else
+  flex_flags := -w
+endif
+CFLAGS_parse-events-flex.o  += $(flex_flags)
+CFLAGS_pmu-flex.o           += $(flex_flags)
+CFLAGS_expr-flex.o          += $(flex_flags)
 CFLAGS_parse-events-bison.o += -DYYENABLE_NLS=0 -w
 CFLAGS_pmu-bison.o          += -DYYENABLE_NLS=0 -DYYLTYPE_IS_TRIVIAL=0 -w
 CFLAGS_expr-bison.o         += -DYYENABLE_NLS=0 -DYYLTYPE_IS_TRIVIAL=0 -w