[libclc] [libclc] Improve nextafter behaviour around zero (PR #127469)

[Matt Arsenault via cfe-commits]

================
@@ -13,19 +13,25 @@
     const UINT_TYPE sign_bit = (UINT_TYPE)1                                    \
                                << (sizeof(INT_TYPE_SCALAR) * 8 - 1);           \
     const UINT_TYPE sign_bit_mask = sign_bit - (UINT_TYPE)1;                   \
-    INT_TYPE ix = CLC_AS_TYPE(INT_TYPE)(x);                                    \
-    UINT_TYPE ax = CLC_AS_TYPE(UINT_TYPE)(ix) & sign_bit_mask;                 \
-    INT_TYPE mx = CLC_AS_TYPE(INT_TYPE)(sign_bit) - ix;                        \
-    mx = CLC_AS_TYPE(INT_TYPE)(ix) < (INT_TYPE)0 ? mx : ix;                    \
-    INT_TYPE iy = CLC_AS_TYPE(INT_TYPE)(y);                                    \
-    UINT_TYPE ay = CLC_AS_TYPE(UINT_TYPE)(iy) & sign_bit_mask;                 \
-    INT_TYPE my = CLC_AS_TYPE(INT_TYPE)(sign_bit) - iy;                        \
-    my = iy < (INT_TYPE)0 ? my : iy;                                           \
+    UINT_TYPE ix = CLC_AS_TYPE(UINT_TYPE)(x);                                  \
+    UINT_TYPE ax = ix & sign_bit_mask;                                         \
+    UINT_TYPE mxu = sign_bit - ix;                                             \
+    INT_TYPE mx = CLC_AS_TYPE(INT_TYPE)(mxu);                                  \
+    mx = CLC_AS_TYPE(INT_TYPE)(ix) < (INT_TYPE)0 ? mx                          \
+                                                 : CLC_AS_TYPE(INT_TYPE)(ix);  \
+    UINT_TYPE iy = CLC_AS_TYPE(UINT_TYPE)(y);                                  \
+    UINT_TYPE ay = iy & sign_bit_mask;                                         \
+    UINT_TYPE myu = sign_bit - iy;                                             \
+    INT_TYPE my = CLC_AS_TYPE(INT_TYPE)(myu);                                  \
+    my = CLC_AS_TYPE(INT_TYPE)(iy) < (INT_TYPE)0 ? my                          \
+                                                 : CLC_AS_TYPE(INT_TYPE)(iy);  \
     INT_TYPE t = mx + (mx < my ? (INT_TYPE)1 : (INT_TYPE)-1);                  \
-    INT_TYPE r = CLC_AS_TYPE(INT_TYPE)(sign_bit) - t;                          \
-    r = t < (INT_TYPE)0 ? r : t;                                               \
+    UINT_TYPE r = sign_bit - CLC_AS_TYPE(UINT_TYPE)(t);                        \
+    r = (t < (INT_TYPE)0 || (t == (INT_TYPE)0 && mx < my))                     \
+            ? r                                                                \
+            : CLC_AS_TYPE(UINT_TYPE)(t);                                       \
     r = __clc_isnan(x) ? ix : r;                                               \
-    r = __clc_isnan(y) ? CLC_AS_TYPE(INT_TYPE)(iy) : r;                        \
+    r = __clc_isnan(y) ? iy : r;                                               \
     r = ((ax | ay) == (UINT_TYPE)0 || ix == iy) ? iy : r;                      \
----------------
arsenm wrote:

```suggestion
    r = ((CLC_AS_TYPE(INT_TYPE)(ax) | CLC_AS_TYPE(INT_TYPE)(ay)) == 0 | ix == iy) ? iy : r;
```




https://github.com/llvm/llvm-project/pull/127469