[Info-vax] C and assembly language

[Stephen Hoffman]

On 2020-12-26 14:04:03 +0000, David Jones said:

> On Wednesday, December 23, 2020 at 11:31:42 AM UTC-5, xyzz... at gmail.com wrote:
>> Consider a broken C routine that is declared "int" but one or more 
>> paths don't have a valid 'return' statement. On Alpha/Itanium, the 
>> value in R0/r8 is often the return value from a prior call to 
>> something. GEM doesn't really want to reuse R0/r8 for any kind of 
>> temporary since it won't live past the next routine call (it will use 
>> them if pushed into a corner). The return without a 'return' just gets 
>> you that prior R0/r8 value. If your 'int' routine is really a 
>> true/false routine, you might have been working for years without any 
>> problems.
>> However, given the few registers on x86 (and how some of the 
>> instructions work), the return register %rax is used in many places for 
>> a short-term temporary. So when the C routine exits without a 'return' 
>> statement, the value in %rax can be very strange.
>> The C program has been broken on every platform. If you would have used 
>> something like /WARNING=ENABLE=QUESTCODE, the compiler can help 
>> identify those paths without a 'return'. You can look for those broken 
>> programs today on Alpha or Itanium. [I've been tempted to promote that 
>> check out of the QUESTCODE category and have it enabled by default. 
>> That decision was done long ago before I was in a position to do 
>> anything to help.]
> 
> DECC's default settings have given warnings about that 
> (%CC-W-MISSINGRETURN) for some time now, though it used to be an 
> aggravating source of bugs.

Discussions of the Clang -Wall (variously with -Wpendant and/or -Wextra 
added) versus the less-than-recommended -Weverything have a long 
history, too.

Clang warnings: 
https://quuxplusone.github.io/blog/2018/12/06/dont-use-weverything/

For those new to Clang: https://nus-cs1010.github.io/2021-s1/clang.html

For OpenVMS, /WARNING=ENABLE=QUESTCODE being lit by default wouldn't 
bother me, as I tend to use that and /STANDARD=C99 or RELAXED lit, and 
a few other switches.

Which usually then evolves into something akin to: /STANDARD=C99 
/WARN=( ENABLE=( NOC99, OBSOLESCENT, DEFUNCT, QUESTCODE, ...), 
DISABLE=(...))

I've found that OpenVMS C code still written with VAXC features tends 
to have latent bugs, too. Code with /STANDARD=VAXC is code that should 
be looked at. Stuff that I'm maintaining that has /STANDARD=VAXC lit 
eventually doesn't have that switch anymore.


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