[PATCH] D85778: More accurately compute the ranges of possible values for +, -, *, &, %.

[Richard Smith - zygoloid via Phabricator via cfe-commits]

rsmith added a comment.

In D85778#2251160 <https://reviews.llvm.org/D85778#2251160>, @zequanwu wrote:

> Hi, this change seems like hits a false positive case in chromium build: https://bugs.chromium.org/p/chromium/issues/detail?id=1124085

That's not a false positive. The code is (simplified):

  int RoundDown(int a, long b) { return a & -b; }

... which is implicitly converting an expression of type `long` to `int`, losing precision. For example, `RoundDown(-1, 0x1'0000'0000)` is -4294967296 prior to the implicit conversion from `long` to `int`. Given that the truncation is presumably intentional (0 at least seems like the least-bad answer for rounding down 0 to a multiple of 2^32 as a 32-bit integer), you can suppress the warning with a cast.


Repository:
  rG LLVM Github Monorepo

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  https://reviews.llvm.org/D85778/new/

https://reviews.llvm.org/D85778