fpu: Handle m68k extended precision denormals properly

Motorola treats denormals with explicit integer bit set as
having unbiased exponent 0, unlike Intel which treats it as
having unbiased exponent 1 (more like all other IEEE formats
that have no explicit integer bit).

Add a flag on FloatFmt to differentiate the behaviour.

Reported-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>

2 files changed, 54 insertions, 1 deletions

diff --git a/tests/tcg/m68k/Makefile.target b/tests/tcg/m68k/Makefile.target
index 1163c7e..6ff214e 100644
--- a/tests/tcg/m68k/Makefile.target
+++ b/tests/tcg/m68k/Makefile.target
@@ -4,7 +4,7 @@
 #
 
 VPATH += $(SRC_PATH)/tests/tcg/m68k
-TESTS += trap
+TESTS += trap denormal
 
 # On m68k Linux supports 4k and 8k pages (but 8k is currently broken)
 EXTRA_RUNS+=run-test-mmap-4096 # run-test-mmap-8192
diff --git a/tests/tcg/m68k/denormal.c b/tests/tcg/m68k/denormal.c
new file mode 100644
index 0000000..20bd8c7
--- /dev/null
+++ b/tests/tcg/m68k/denormal.c
@@ -0,0 +1,53 @@
+/*
+ * Test m68k extended double denormals.
+ */
+
+#include <stdio.h>
+#include <stdint.h>
+
+#define TEST(X, Y)  { X, Y, X * Y }
+
+static volatile long double test[][3] = {
+    TEST(0x1p+16383l, 0x1p-16446l),
+    TEST(0x1.1p-8223l, 0x1.1p-8224l),
+    TEST(1.0l, 0x1p-16383l),
+};
+
+#undef TEST
+
+static void dump_ld(const char *label, long double ld)
+{
+    union {
+        long double  d;
+        struct {
+            uint32_t exp:16;
+            uint32_t space:16;
+            uint32_t h;
+            uint32_t l;
+        };
+    } u;
+
+    u.d = ld;
+    printf("%12s: % -27La 0x%04x 0x%08x 0x%08x\n", label, u.d, u.exp, u.h, u.l);
+}
+
+int main(void)
+{
+    int i, n = sizeof(test) / sizeof(test[0]), err = 0;
+
+    for (i = 0; i < n; ++i) {
+        long double x = test[i][0];
+        long double y = test[i][1];
+        long double build_mul = test[i][2];
+        long double runtime_mul = x * y;
+
+        if (runtime_mul != build_mul) {
+            dump_ld("x", x);
+            dump_ld("y", y);
+            dump_ld("build_mul", build_mul);
+            dump_ld("runtime_mul", runtime_mul);
+            err = 1;
+        }
+    }
+    return err;
+}