Issue |
93288
|
Summary |
Addressing `-Wimplicit-float-conversion` warning might lead to precision loss
|
Labels |
new issue
|
Assignees |
|
Reporter |
firewave
|
Disclaimer: My knowledge about floating-point precision is quite limited and I based most of my interpretations on the linked discussion. I came across this by accident and was wondering if there was something of the short-coming that could be detected by tooling. While looking into this I encountered this apparent issue.
This is based on the discussion in https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28336.
According to that discussion the floating-point result of `48 * (1 / 255)` should be `0.18823529779911041259765625`. But if you simply write that it will lead to a compiler warning.
```cpp
float f = 48 * (1.0 / 255.0);
```
```
warning: implicit conversion loses floating-point precision: 'double' to 'float' [-Wimplicit-float-conversion]
12 | float f = 48 * (1.0 / 255.0);
```
The obvious fix looks like adding the `f` suffix to the literals.
```cpp
float f = 48 * (1.0f / 255.0f);
```
That fixes the warning but it changes the result to `0.1882353127002716064453125` which is supposedly less precise.
It took me quite a while until I finally came up with a solution which did not produce any warnings as well as the expected result.
```cpp
float f = (float)(48 * (1.0 / 255.0));
```
As there are several ways to suppress (I do not consider it a fix when it changes the result/behavior) this warning seems likely to cause more harm than good so I thought it might be worth raising this issue.
See https://godbolt.org/z/crnKhzsec for all my attempts to mitigate this warning.
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