probinson added a comment.
In D91734#2432988 <https://reviews.llvm.org/D91734#2432988>, @dblaikie wrote:
> Yeah, it boils down to something like this:
>
> void f1(const char*, const char*) { }
> int main() {
> char prog[1];
>
> f1(prog,
> prog);
> }
>
> Without the patch, you step to the 'f1' line, and then step into or over the
> function call (step or next).
> But with these patches applied - you step to the 'f1(' line, then the
> 'prog);' line, then back to the 'f1(' line, then into/over the function call.
>
> Again, could be acceptable - if those arguments had specific non-trivial code
> in them (like other function calls) you'd certainly get that step
> forward/backward behavior - but it's notewhorthy that this is change would
> make more cases like that & it'd be good to understand why/if that's a good
> thing overall.
If you dump the IR you should see two GEP instructions, one for each actual
parameter expression. Prior to the patch, FastISel found the second one
computed the same value as the first one, and reused the value (in this case,
the result of `lea`). This is one of the 5% of cases where a value is being
reused across instructions, with the HEAD compiler. After the patch, we flush
the local map after each GEP, so the second one gets its own separate `lea`
(with its own source location). So, you're stepping to the first actual, then
to the second, then to the call. Hope that answers the "why" question.
Whether it's a "good" thing... that path is fraught with peril. Looking at it
somewhat abstractly (as I am sometimes wont to do), Clang is making choices to
associate the source location of each parameter with the IR instructions to
compute that parameter, and by flushing the map on every IR instruction, LLVM
is more faithfully representing that in the final object. This is -O0 after
all, where a more faithful representation is more likely to be more debuggable.
gcc is making entirely different choices, lumping all parameter-prep
instructions in with the function call itself. I'm hard pressed to say one is
better than the other; they are different. Coke and Pepsi; people may have
preferences, but that's not the same as being higher on a good-ness scale.
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