[SIL Diagnostics] Warn when assigning an integer literal to a variable of floating-point type results in loss of precision. #17274

ravikandhadai · 2018-06-16T02:27:14Z

This feature was requested in the radar issue: rdar://15224041.

ravikandhadai · 2018-06-16T02:29:44Z

The warnings are disabled when the conversion happens through explicit constructor call. E.g.

$ let f3: Float = 16777217
warning: '16777217' loses precision during conversion to 'Float'
$ Float(16777217) // no warning

ravikandhadai · 2018-06-16T02:30:01Z

@swift-ci please smoke test

stephentyrone · 2018-06-16T20:48:13Z

Would it be more clear if we tell the user what value actually results? Maybe something along the lines of: "Warning: '16777217' is not exactly representable as 'Float'; 'f3' has value '16777216'.

ravikandhadai · 2018-06-19T04:38:08Z

@swift-ci please smoke test

ravikandhadai · 2018-06-19T04:41:58Z

@stephentyrone Sounds good to me. I have pushed a commit that prints the new value in the warning messages. E.g.

$ let f3: Float = 16777217
warning: '16777217' is not exactly representable as 'Float'; it becomes '16777216'

stephentyrone · 2018-06-19T16:36:19Z

Nice! If you update the PR with conflicts resolved, I can review later today or tomorrow.

jrose-apple · 2018-06-19T17:13:19Z

test/SILOptimizer/diagnostic_constant_propagation.swift

-  // 2^128 * (2^25-1)/2^25
+  var _ /*float32_max*/                     : Float32 = (340282346638528859811704183484516925440) // in hexfloat = 0x1.fffffep127
+
+  var _ /*float32_near_max_but_imprecise*/  : Float32 = (340282326356119256160033759537265639425) // expected-warning {{'340282326356119256160033759537265639425' is not exactly representable as 'Float32' (aka 'Float'); it becomes '3.40282326E+38'}}


We probably want to be printing these with full precision, to make it clear what the difference is.

@jrose-apple Yeah sure, let me make that change.

ravikandhadai · 2018-06-19T23:24:02Z

@swift-ci please smoke test

ravikandhadai · 2018-06-19T23:27:48Z

@jrose-apple Done. Now the new float value is displayed like an integer to make the imprecision obvious E.g.
var _ : Float32 = (340282326356119256160033759537265639425)
> warning: '340282326356119256160033759537265639425' is not exactly representable as 'Float32' (aka 'Float'); it becomes '340282326356119256160033759537265639424'

@stephentyrone I have fixed the merged conflicts. Thanks for your help with this.

ravikandhadai · 2018-06-20T00:06:59Z

@swift-ci please smoke test

stephentyrone

LGTM, thanks!

ravikandhadai · 2018-06-20T22:56:24Z