[PATCH] D147875: [clang][Diagnostics] Show line numbers when printing code snippets

[Aaron Puchert via Phabricator via cfe-commits]

aaronpuchert added inline comments.


================
Comment at: clang/lib/Frontend/TextDiagnostic.cpp:1121-1138
+static unsigned getNumDisplayWidth(unsigned N) {
+  if (N < 10)
+    return 1;
+  if (N < 100)
+    return 2;
+  if (N < 1'000)
+    return 3;
----------------
aaronpuchert wrote:
> efriedma wrote:
> > aaron.ballman wrote:
> > > jrtc27 wrote:
> > > > kwk wrote:
> > > > > This function screamed at me to be generalized so I gave it a try: https://gist.github.com/kwk/7e408065ea291e49fea4a83cf90a9cdf
> > > > I don't think I want floating point arithmetic in my compiler... Something like:
> > > > 
> > > > ```
> > > >     unsigned L, M;
> > > >     for (L = 1U, M = 10U; N >= M && M != ~0U; ++L)
> > > >         M = (M > ((~0U) / 10U)) ? (~0U) : (M * 10U);
> > > >     
> > > >     return (L);
> > > > ```
> > > > 
> > > > is the generalised form (without all the redundant parentheses I added during debugging).
> > > Cleaned up a bit:
> > > ```
> > > constexpr unsigned getNumDisplayWidth(unsigned N) {
> > >   unsigned L = 1U, M = 10U;
> > >   constexpr unsigned Upper = ~0U / 10U;
> > >   for (; N >= M && M != ~0U; ++L)
> > >     M = M > Upper ? ~0U : M * 10U;
> > >   return L;
> > > }
> > > ```
> > > https://godbolt.org/z/szTYsEM4v
> > Slightly less efficient, but easier to read:
> > 
> > ```
> > unsigned getNumDisplayWidth(unsigned N) {
> >   unsigned Width = 1;
> >   for (; N >= 10; ++Width)
> >     N /= 10;
> >   return Width;
> > }
> > ```
> I'd suggest to replace `~0U` by `std::numeric_limits<unsigned>::max()`. And if we're looking at `<limits>`, why not ask it directly how far we need to go?
> ```
> static unsigned getNumDisplayWidth(unsigned N) {
>   unsigned L = 1u, M = 10u;
>   constexpr unsigned Max = std::numeric_limits<unsigned>::digits10 + 1;
>   for (; M <= N && L != Max; ++L)
>     M *= 10u;
>   return L;
> }
> ```
> This will let the overflow happen, but that should be fine for unsigned arithmetic. Without overflow:
> ```
> static unsigned getNumDisplayWidth(unsigned N) {
>   unsigned L = 1u, M = 10u;
>   while (M <= N && L++ != std::numeric_limits<unsigned>::digits10)
>     M *= 10u;
>   return L;
> }
> ```
> The trick is that the increment is done even if the comparison fails and we exit the loop.
Though pre-increment also does it if we change the right-hand side:
```
  while (M <= N && ++L != std::numeric_limits<unsigned>::digits10 + 1)
    M *= 10u;
```


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  https://reviews.llvm.org/D147875/new/

https://reviews.llvm.org/D147875