fix #10305 nim cpp is now nan-correct at CT (#10310)

* fix #10305 nim cpp is now nan-correct at CT
* add example where simply `nim cpp -d:release` would exhibit nan bug

compiler/extccomp.nim

@@ -63,8 +63,8 @@ compiler gcc:
   result = (
     name: "gcc",
     objExt: "o",
-    optSpeed: " -O3 -ffast-math ",
+    optSpeed: " -O3 ",
-    optSize: " -Os -ffast-math ",
+    optSize: " -Os ",
     compilerExe: "gcc",
     cppCompiler: "g++",
     compileTmpl: "-c $options $include -o $objfile $file",
@@ -88,8 +88,8 @@ compiler nintendoSwitchGCC:
   result = (
     name: "switch_gcc",
     objExt: "o",
-    optSpeed: " -O3 -ffast-math ",
+    optSpeed: " -O3 ",
-    optSize: " -Os -ffast-math ",
+    optSize: " -Os ",
     compilerExe: "aarch64-none-elf-gcc",
     cppCompiler: "aarch64-none-elf-g++",
     compileTmpl: "-w -MMD -MP -MF $dfile -c $options $include -o $objfile $file",

doc/nimc.rst

@@ -347,6 +347,9 @@ complete list.
 Define                   Effect
 ======================   =========================================================
 ``release``              Turns off runtime checks and turns on the optimizer.
+                         More aggressive optimizations are possible, eg:
+                         ``--passC:-ffast-math`` (but see issue #10305)
+                         ``--stacktrace:off``
 ``useWinAnsi``           Modules like ``os`` and ``osproc`` use the Ansi versions
                          of the Windows API. The default build uses the Unicode
                          version.

tests/float/tfloatnan.nim

@@ -14,3 +14,31 @@ echo "Nim: ", f32, " (float)"
 
 let f64: float64 = NaN
 echo "Nim: ", f64, " (double)"
+
+block: # issue #10305
+  # with `-O3 -ffast-math`, generated C/C++ code is not nan compliant
+  # user can pass `--passC:-ffast-math` if he doesn't care.
+  proc fun() =
+    # this was previously failing at compile time with a nim compiler
+    # that was compiled with `nim cpp -d:release`
+    let a1 = 0.0
+    let a = 0.0/a1
+    let b1 = a == 0.0
+    let b2 = a == a
+    doAssert not b1
+    doAssert not b2
+
+  proc fun2(i: int) =
+    # this was previously failing simply with `nim cpp -d:release`; the
+    # difference with above example is that optimization (const folding) can't
+    # take place in this example to hide the non-compliant nan bug.
+    let a = 0.0/(i.float)
+    let b1 = a == 0.0
+    let b2 = a == a
+    doAssert not b1
+    doAssert not b2
+
+  static: fun()
+  fun()
+  fun2(0)
+