[dmd-internals] Errors emitted by the optimizer and the as-if rule

Iain Buclaw ibuclaw at ubuntu.com
Sun Aug 19 01:25:13 PDT 2012

On 18 August 2012 20:01, David Nadlinger <code at klickverbot.at> wrote:
> On Sat, Aug 18, 2012 at 8:38 PM, kenji hara <k.hara.pg at gmail.com> wrote:
>> My answer is: optimizer doesn't affect the result of semantic analysis.
>> In other words, flow analysis by the optimizer also doesn't affect to
>> the compilation.
> I think you ares  completely missing the point missing the point. Flow
> analysis performed by the optimizer _does_ affect compilation, because
> it leads to code being treated as illegal which would be perfectly
> fine otherwise. How much bigger of an effect on language semantics can
> there be?
>> The "used before set"  error disappears if you don't specify -O.
>> Therefore it is backend optimizer's work, and is not a part of the
>> spec.
> Which is a big problem. Whether a program compiles or not should be
> clear from the spec, and not be dependent on the compiler writer's
> mood. If the optimizer detects undefined behavior, it can enable all
> sorts of crazy optimizations – for example, LLVM routinely optimizes
> down big chunks of code with just a trap if it detects undefined
> values –, but it can't change language semantics: if the spec says
> that something is undefined behavior, executing the code results in,
> well, undefined behavior, but the code is still perfectly legal, the
> compiler should _not_ be allowed to generate an error.
> To illustrate my point, consider the following example. It contains
> code which exhibits undefined behavior, but since this code is never
> invoked, there should be no problem:
> ---
> void foo() {
>    int a = void;
>    import std.stdio;
>    writeln(a);
> }
> void main() {}
> ----
> David

You can only be certain that a function is never invoked unless it is
marked 'static' IMO.  My opinion is that it should warn you anyway, as
it is potentially buggy code, even if unused.

Iain Buclaw

*(p < e ? p++ : p) = (c & 0x0f) + '0';

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