d: Suboptimal codegen for __builtin_expect(cond, false)

Since PR96435, both boolean objects and expressions have been evaluated
in the following way.

    (*(ubyte*)&obj_or_expr) & 1

It has been noted that sometimes this can cause the back-end to optimize
in non-obvious ways - in particular with __builtin_expect.

This @safe feature is now restricted to just when reading the value of a
bool field that comes from a union.

	PR d/110359

gcc/d/ChangeLog:

	* d-convert.cc (convert_for_rvalue): Only apply the @safe boolean
	conversion to boolean fields of a union.
	(convert_for_condition): Call convert_for_rvalue in the default case.

gcc/testsuite/ChangeLog:

	* gdc.dg/pr110359.d: New test.

1 files changed, 22 insertions, 0 deletions

diff --git a/gcc/testsuite/gdc.dg/pr110359.d b/gcc/testsuite/gdc.dg/pr110359.d
new file mode 100644
index 0000000..bf69201
--- /dev/null
+++ b/gcc/testsuite/gdc.dg/pr110359.d
@@ -0,0 +1,22 @@
+// https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110359
+// { dg-do compile }
+// { dg-options "-fdump-tree-original" }
+double pow(in double x, in ulong p)
+{
+    import gcc.builtins : __builtin_expect;
+    if (__builtin_expect(p == 0, false))
+        return 1;
+    if (__builtin_expect(p == 1, false))
+        return x;
+
+    double s = x;
+    double v = 1;
+    for (ulong i = p; i > 1; i >>= 1)
+    {
+        v = (i & 0x1) ? s * v : v;
+        s = s * s;
+    }
+    return v * s;
+}
+// { dg-final { scan-tree-dump "if \\(__builtin_expect \\(p == 0, 0\\) != 0\\)" "original" } }
+// { dg-final { scan-tree-dump "if \\(__builtin_expect \\(p == 1, 0\\) != 0\\)" "original" } }