#4042 Weird uninitialized variable warning on local function prototype

[Maarten Brock]

Tcc regression test 105_local_extern.c outputs a strange warning about an auto variable f where there is no variable definition at all.

cases/../tcc/105_local_extern.c:3: warning 84: 'auto' variable 'f' may be used before initialization

[Christopher Bazley]

I have a fix for this that I am testing.

[Christopher Bazley]

I am attaching a proposed implementation of this feature request, created with assistance from Codex. I hope that it will prove acceptable when reviewed by SDCC's maintainers.

1. The first version made an ordinary block-scope function declaration
   implicitly extern in yyparse() declaration action -> prepareDeclarationSymbol(). It applied only when SPEC_SCLS was
   S_FIXED, but that was too restrictive because SDCC also stores
   target-specific address spaces such as __xdata in SPEC_SCLS. None of
   the tests produced by Codex revealed that.

2. A handwritten extra test of __xdata int f(void) at block scope
   showed that such declarations could still be mistaken for automatic
   objects, so the bogus S_FIXED restriction was removed.

3. The new test also exposed a deeper problem with --stack-auto: even
   explicitly extern declarations such as extern __xdata int f(void)
   were subject to checks intended for automatic objects. The next
   version therefore excluded functions from several automatic-object
   checks in a later call chain: yyparse() function_definition action -> createFunction() -> processBlockVars() -> allocVariables() -> checkDecl() -> checkSClass(). That worked, but further complicated
   already-repetitious if conditions in the modified function and
   described the exception rather than the rule.

4. I investigated whether it would be practical to reduce repetition
   by use of an early-return from checkSClass() without significant
   refactoring. It was not.

5. The final approach uses IS_AUTO(sym) in checkSClass() instead.
   IS_AUTO(sym) means that the symbol is at block scope and is neither
   static nor extern. prepareDeclarationSymbol() has already marked an
   ordinary block-scope function declaration as extern, so IS_AUTO(sym)
   excludes it without a separate function check.

6. Using IS_AUTO(sym) consistently also fixes block-scope extern
   object declarations: SDCC 4.6.0 incorrectly rejects a declaration such as extern __xdata int object when compiled with options -mmcs51 --stack-auto, because it wrongly sets E_AUTO_ASSUMED. After confirming
   that fact, I added another new handwritten test to the set of new
   tests.

I did the following testing of the patch:

• complete diagnostic-validation results (support/valdiag) for the default targets.
• complete runtime regression (support/regression) results for ucz80.

i.e.

make -j2
make -C support/regression clean-results
make -C support/regression -j2 test-ucz80
make -C support/valdiag clean
make -C support/valdiag -j2

Summary for 'ucz80': 0 failures, 36740 tests, 6407 test cases, 7347620 bytes, 1613638336 ticks
Summary for 'mcs51': 0 failures, 642 tests, 364 test cases, 0 bytes, 0 ticks
Summary for 'mcs51-large': 0 failures, 640 tests, 364 test cases, 0 bytes, 0 ticks
Summary for 'mcs51-stack-auto': 0 failures, 641 tests, 364 test cases, 0 bytes, 0 ticks
Summary for 'ds390': 0 failures, 628 tests, 364 test cases, 0 bytes, 0 ticks
Summary for 'z80': 0 failures, 631 tests, 364 test cases, 0 bytes, 0 ticks
Summary for 'z180': 0 failures, 630 tests, 364 test cases, 0 bytes, 0 ticks
Summary for 'r2k': 0 failures, 630 tests, 364 test cases, 0 bytes, 0 ticks
Summary for 'r4k': 0 failures, 630 tests, 364 test cases, 0 bytes, 0 ticks
Summary for 'sm83': 0 failures, 630 tests, 364 test cases, 0 bytes, 0 ticks
Summary for 'tlcs90': 0 failures, 630 tests, 364 test cases, 0 bytes, 0 ticks
Summary for 'hc08': 0 failures, 629 tests, 364 test cases, 0 bytes, 0 ticks
Summary for 's08': 0 failures, 629 tests, 364 test cases, 0 bytes, 0 ticks
Summary for 'mos6502': 0 failures, 629 tests, 364 test cases, 0 bytes, 0 ticks
Summary for 'stm8': 0 failures, 632 tests, 364 test cases, 0 bytes, 0 ticks
Summary for 'f8': 0 failures, 628 tests, 364 test cases, 0 bytes, 0 ticks
Summary for 'pdk13': 0 failures, 631 tests, 364 test cases, 0 bytes, 0 ticks
Summary for 'pdk14': 0 failures, 629 tests, 364 test cases, 0 bytes, 0 ticks
Summary for 'pdk15': 0 failures, 627 tests, 364 test cases, 0 bytes, 0 ticks