Revamp of GC parameters

More uniformity when handling GC parameters + avoid divisions by 100
when applying them.

lgc.h

@@ -8,6 +8,9 @@
 #define lgc_h
 
 
+#include <stddef.h>
+
+
 #include "lobject.h"
 #include "lstate.h"
 
@@ -122,20 +125,18 @@
 
 
 /* Default Values for GC parameters */
-#define LUAI_GENMAJORMUL         100
-#define LUAI_GENMINORMUL         20
+
+/* generational */
+
+#define LUAI_GENMAJORMUL         100	/* major multiplier */
+#define LUAI_GENMINORMUL         20	/* minor multiplier */
+
+/* incremental */
 
 /* wait memory to double before starting new cycle */
 #define LUAI_GCPAUSE    200
 
-/*
-** some gc parameters are stored divided by 4 to allow a maximum value
-** up to 1023 in a 'lu_byte'.
-*/
-#define getgcparam(p)	((p) * 4)
-#define setgcparam(p,v)	((p) = (v) / 4)
-
-#define LUAI_GCMUL      300
+#define LUAI_GCMUL      300	/* step multiplier */
 
 /* how many objects to allocate before next GC step (log2) */
 #define LUAI_GCSTEPSIZE 8      /* 256 objects */
@@ -149,6 +150,29 @@
 #define GCSTPCLS	4  /* bit true when closing Lua state */
 #define gcrunning(g)	((g)->gcstp == 0)
 
+/*
+** Macros to set and apply GC parameters. GC parameters are given in
+** percentage points, but are stored as lu_byte. To reduce their
+** values and avoid repeated divisions by 100, these macros store
+** the original parameter multiplied by 2^n and divided by 100.
+** To apply them, the value is divided by 2^n (a shift) and then
+** multiplied by the stored parameter, yielding
+** value / 2^n * (original parameter * 2^n / 100), or approximately
+** (value * original parameter / 100).
+**
+** For most parameters, which are typically larger than 100%, 2^n is
+** 16 (2^4), allowing maximum values up to 1599.  For the minor
+** multiplier, which is typically smaller, 2^n is 64 (2^6) to allow more
+** precision.
+*/
+#define gcparamshift(p)  \
+  (offsetof(global_State, p) == offsetof(global_State, genminormul) ? 6 : 4)
+
+#define setgcparam(g,p,v)  \
+	(g->p = (cast_uint(v) << gcparamshift(p)) / 100u)
+#define applygcparam(g,p,v)	(((v) >> gcparamshift(p)) * g->p)
+
+
 
 /*
 ** Does one step of collection when debt becomes positive. 'pre'/'pos'