SPARC NCG: Add a comment explaining why we can't used a pinned reg for gct

Can't use windowed regs because the window moves during a function
call. Can't use the global regs because they're reserved for other purposes.
parent fd2bd22e
......@@ -209,10 +209,9 @@ extern gc_thread **gc_threads;
#define SET_GCT(to) gct = (to)
#if defined(sparc_HOST_ARCH) || (defined(i386_HOST_ARCH) && defined(linux_HOST_OS))
// Don't use REG_base or R1 for gct on SPARC because they're getting clobbered
// by something else. Not sure what yet. -- BL 2009/01/03
#if (defined(i386_HOST_ARCH) && defined(linux_HOST_OS))
// Using __thread is better than stealing a register on x86/Linux, because
// we have too few registers available. In my tests it was worth
// about 5% in GC performance, but of course that might change as gcc
......@@ -221,6 +220,22 @@ extern gc_thread **gc_threads;
extern __thread gc_thread* gct;
#define DECLARE_GCT __thread gc_thread* gct;
#elif defined(sparc_TARGET_ARCH)
// On SPARC we can't pin gct to a register. Names like %l1 are just offsets
// into the register window, which change on each function call.
//
// There are eight global (non-window) registers, but they're used for other purposes.
// %g0 -- always zero
// %g1 -- volatile over function calls, used by the linker
// %g2-%g3 -- used as scratch regs by the C compiler (caller saves)
// %g4 -- volatile over function calls, used by the linker
// %g5-%g7 -- reserved by the OS
extern __thread gc_thread* gct;
#define DECLARE_GCT __thread gc_thread* gct;
#elif defined(REG_Base) && !defined(i386_HOST_ARCH)
// on i386, REG_Base is %ebx which is also used for PIC, so we don't
// want to steal it
......@@ -228,11 +243,13 @@ extern __thread gc_thread* gct;
GLOBAL_REG_DECL(gc_thread*, gct, REG_Base)
#define DECLARE_GCT /* nothing */
#elif defined(REG_R1)
GLOBAL_REG_DECL(gc_thread*, gct, REG_R1)
#define DECLARE_GCT /* nothing */
#elif defined(__GNUC__)
extern __thread gc_thread* gct;
......
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