make -d:nimFpRoundtrips work consistently in vm vs rt, fix #18400, etc (#18531)

* compiler/vmhooks: add getVar to allow vmops with var params
* addFloat vmops with var param
* cgen now renders float32 literals in c backend using roundtrip float to string

tests/float/nim.cfg

@@ -1 +0,0 @@
--d:nimFpRoundtrips

tests/float/tfloats.nim

@@ -1,13 +1,14 @@
 discard """
+  matrix: "-d:nimFpRoundtrips; -u:nimFpRoundtrips"
   targets: "c cpp js"
 """
-# disabled: "windows"
 
 #[
 xxx merge all or most float tests into this file
 ]#
 
 import std/[fenv, math, strutils]
+import stdtest/testutils
 
 proc equalsOrNaNs(a, b: float): bool =
   if isNaN(a): isNaN(b)
@@ -62,27 +63,103 @@ template main =
     reject "1_.0"
     reject "1.0_"
 
-  block: # bug #18148
-    var a = 1.1'f32
-    doAssert $a == "1.1", $a # was failing
+  block: # bugs mentioned in https://github.com/nim-lang/Nim/pull/18504#issuecomment-881635317
+    block: # example 1
+      let a = 0.1+0.2
+      doAssert a != 0.3
+      when defined(nimFpRoundtrips):
+        doAssert $a == "0.30000000000000004"
+      else:
+        whenRuntimeJs: discard
+        do: doAssert $a == "0.3"
+    block: # example 2
+      const a = 0.1+0.2
+      when defined(nimFpRoundtrips):
+        doAssert $($a, a) == """("0.30000000000000004", 0.30000000000000004)"""
+      else:
+        whenRuntimeJs: discard
+        do: doAssert $($a, a) == """("0.3", 0.3)"""
+    block: # example 3
+      const a1 = 0.1+0.2
+      let a2 = a1
+      doAssert a1 != 0.3
+      when defined(nimFpRoundtrips):
+        doAssert $[$a1, $a2] == """["0.30000000000000004", "0.30000000000000004"]"""
+      else:
+        whenRuntimeJs: discard
+        do: doAssert $[$a1, $a2] == """["0.3", "0.3"]"""
 
-proc runtimeOnlyTests =
-  # enable for 'static' once -d:nimFpRoundtrips became the default
-  block: # bug #7717
-    proc test(f: float) =
-      let f2 = $f
-      let f3 = parseFloat(f2)
-      doAssert equalsOrNaNs(f, f3), $(f, f2, f3)
+  when defined(nimFpRoundtrips):
+    block: # bug #18148
+      var a = 1.1'f32
+      doAssert $a == "1.1", $a # was failing
 
-    test 1.0 + epsilon(float64)
-    test 1000000.0000000123
-    test log2(100000.0)
-    test maximumPositiveValue(float32)
-    test maximumPositiveValue(float64)
-    test minimumPositiveValue(float32)
-    test minimumPositiveValue(float64)
+    block: # bug #18400
+      block:
+        let a1 = 0.1'f32
+        let a2 = 0.2'f32
+        let a3 = a1 + a2
+        var s = ""
+        s.addFloat(a3)
+        whenVMorJs: discard # xxx refs #12884
+        do:
+          doAssert a3 == 0.3'f32
+          doAssert $a3 == "0.3"
+
+      block:
+        let a1 = 0.1
+        let a2 = 0.2
+        let a3 = a1 + a2
+        var s = ""
+        s.addFloat(a3)
+        doAssert a3 != 0.3
+        doAssert $a3 == "0.30000000000000004"
+
+      block:
+        var s = [-13.888888'f32]
+        whenRuntimeJs: discard
+        do:
+          doAssert $s == "[-13.888888]"
+          doAssert $s[0] == "-13.888888"
+
+    block: # bug #7717
+      proc test(f: float) =
+        let f2 = $f
+        let f3 = parseFloat(f2)
+        doAssert equalsOrNaNs(f, f3), $(f, f2, f3)
+      test 1.0 + epsilon(float64)
+      test 1000000.0000000123
+      test log2(100000.0)
+      test maximumPositiveValue(float32)
+      test maximumPositiveValue(float64)
+      test minimumPositiveValue(float32)
+      test minimumPositiveValue(float64)
+
+    block: # bug #12884
+      block: # example 1
+        const x0: float32 = 1.32
+        let x1 = 1.32
+        let x2 = 1.32'f32
+        var x3: float32 = 1.32
+        doAssert $(x0, x1, x2, x3) == "(1.32, 1.32, 1.32, 1.32)"
+      block: # example https://github.com/nim-lang/Nim/issues/12884#issuecomment-564967962
+        let x = float(1.32'f32)
+        when nimvm: discard # xxx prints 1.3
+        else:
+          when not defined(js):
+            doAssert $x == "1.3200000524520874"
+        doAssert $1.32 == "1.32"
+        doAssert $1.32'f32 == "1.32"
+        let x2 = 1.32'f32
+        doAssert $x2 == "1.32"
+      block:
+        var x = 1.23456789012345'f32
+        when nimvm:
+          discard # xxx, refs #12884
+        else:
+          when not defined(js):
+            doAssert x == 1.2345679'f32
+            doAssert $x == "1.2345679"
 
 static: main()
 main()
-
-runtimeOnlyTests()