Don't commute negation with float comparison (#1470)

Floating-point comparisons such as `<` and `>=` are always false when one of their arguments is NaN. For this reason, it is incorrect to simplify `not (x < y)` as `x >= y` if this is a FP comparison. This wrong optimization was introduced in 4.06.0 and is being reverted by this commit.
2017-11-21 17:38:18 +00:00 · 2017-11-21 17:38:18 +00:00 · c6f3a00b31
parent 7f5f82dc9c
commit c6f3a00b31
4 changed files with 10 additions and 2 deletions
--- a/3
+++ b/3
@ -80,6 +80,9 @@ Working version
  (Luc Maranget, review by Thomas Refis and Gabriel Scherer, report
  by Abdelraouf Ouadjaout and Thibault Suzanne)

+- GPR#1470: Don't commute negation with float comparison
+  (Leo White, review by Xavier Leroy)
+
 OCaml 4.06.0 (3 Nov 2017):
 --------------------------

--- a/asmcomp/cmmgen.ml
+++ b/asmcomp/cmmgen.ml
@ -292,8 +292,6 @@ let mk_not dbg cmm =
          tag_int (Cop(Ccmpi (negate_comparison cmp), [c1; c2], dbg'')) dbg'
      | Cop(Ccmpa cmp, [c1; c2], dbg'') ->
          tag_int (Cop(Ccmpa (negate_comparison cmp), [c1; c2], dbg'')) dbg'
-      | Cop(Ccmpf cmp, [c1; c2], dbg'') ->
-          tag_int (Cop(Ccmpf (negate_comparison cmp), [c1; c2], dbg'')) dbg'
      | _ ->
        (* 0 -> 3, 1 -> 1 *)
        Cop(Csubi, [Cconst_int 3; Cop(Clsl, [c; Cconst_int 1], dbg)], dbg)
--- a/testsuite/tests/basic-float/float_compare.ml
+++ b/testsuite/tests/basic-float/float_compare.ml
@ -0,0 +1,6 @@
+
+let compare_nan () =
+  not (nan < 0.0)
+[@@inline never]
+
+let x = print_endline (string_of_bool (compare_nan ()))
--- a/testsuite/tests/basic-float/float_compare.reference
+++ b/testsuite/tests/basic-float/float_compare.reference
@ -0,0 +1 @@
+true