Instead of a separate context-switch flag, set HpLim to zero

This reduces the latency between a context-switch being triggered and
the thread returning to the scheduler, which in turn should reduce the
cost of the GC barrier when there are many cores.

We still retain the old context_switch flag which is checked at the
end of each block of allocation.  The idea is that setting HpLim may
fail if the the target thread is modifying HpLim at the same time; the
context_switch flag is a fallback.  It also allows us to "context
switch soon" without forcing an immediate switch, which can be costly.

2 files changed, 10 insertions, 30 deletions

diff --git a/includes/MachRegs.h b/includes/MachRegs.h
index adc132bfe8..f51f782806 100644
--- a/includes/MachRegs.h
+++ b/includes/MachRegs.h
@@ -95,7 +95,6 @@
    s0	$9	Sp
    s2	$11	SpLim
    s3	$12	Hp   
-   s4	$13	HpLim
    t8	$22	NCG_reserved
    t12	$27	NCG_reserved
    -------------------------------------------------------------------------- */
@@ -134,7 +133,6 @@
 #  define REG_SpLim     11
 
 #  define REG_Hp	12
-#  define REG_HpLim	13
   
 #  define NCG_Reserved_I1 22
 #  define NCG_Reserved_I2 27
@@ -189,7 +187,6 @@
 #define REG_SpLim    	r6
 
 #define REG_Hp	    	r7
-#define REG_HpLim	r8
 
 #define NCG_Reserved_I1 r28
 #define NCG_Reserved_I2	r29
@@ -215,7 +212,7 @@
    esi     R1
    edi     Hp
 
-   Leaving SpLim, and HpLim out of the picture.
+   Leaving SpLim out of the picture.
    -------------------------------------------------------------------------- */
 
 
@@ -284,12 +281,12 @@
 #define REG_Sp    rbp
 #define REG_Hp    r12
 #define REG_R1    rbx
-#define REG_R2    rsi
-#define REG_R3    rdi
-#define REG_R4    r8
-#define REG_R5    r9
-#define REG_SpLim r14
-#define REG_HpLim r15
+#define REG_R2    r14
+#define REG_R3    rsi
+#define REG_R4    rdi
+#define REG_R5    r8
+#define REG_R6    r9
+#define REG_SpLim r15
 
 #define REG_F1    xmm1
 #define REG_F2    xmm2
@@ -299,10 +296,10 @@
 #define REG_D1    xmm5
 #define REG_D2    xmm6
 
-#define CALLER_SAVES_R2
 #define CALLER_SAVES_R3
 #define CALLER_SAVES_R4
 #define CALLER_SAVES_R5
+#define CALLER_SAVES_R6
 
 #define CALLER_SAVES_F1
 #define CALLER_SAVES_F2
@@ -312,7 +309,7 @@
 #define CALLER_SAVES_D1
 #define CALLER_SAVES_D2
 
-#define MAX_REAL_VANILLA_REG 5
+#define MAX_REAL_VANILLA_REG 6
 #define MAX_REAL_FLOAT_REG   4
 #define MAX_REAL_DOUBLE_REG  2
 #define MAX_REAL_LONG_REG    0
@@ -361,7 +358,6 @@
 #define REG_SpLim	d3
 
 #define REG_Hp	    	d4
-#define REG_HpLim       d5
 
 #define REG_R1    	a5
 #define REG_R2    	d6
@@ -425,7 +421,6 @@
 #define REG_SpLim    	21
 
 #define REG_Hp	    	22
-#define REG_HpLim	23
 
 #define REG_Base	30
 
@@ -500,7 +495,6 @@
 #define REG_SpLim    	r24
 
 #define REG_Hp	    	r25
-#define REG_HpLim	r26
 
 #define REG_Base        r27
 
@@ -543,7 +537,6 @@
 #define REG_SpLim	loc26
 
 #define REG_Hp		loc27
-#define REG_HpLim	loc28
 
 #endif /* ia64 */
 
@@ -597,7 +590,6 @@
      %i1        Base
      %i2        SpLim
      %i3        Hp
-     %i4        HpLim
      %i5        R6
      %i6                    C frame ptr
      %i7                    C ret addr
@@ -666,7 +658,6 @@
 #define REG_SpLim    	i2
 
 #define REG_Hp	    	i3
-#define REG_HpLim	i4
 
 #define REG_Base	i1
 
diff --git a/includes/Regs.h b/includes/Regs.h
index 45f9149af5..29c04b302f 100644
--- a/includes/Regs.h
+++ b/includes/Regs.h
@@ -133,7 +133,6 @@ typedef struct StgRegTable_ {
 #define SAVE_SpLim    	    (CurrentTSO->splim)
 
 #define SAVE_Hp	    	    (BaseReg->rHp)
-#define SAVE_HpLim	    (BaseReg->rHpLim)
 
 #define SAVE_CurrentTSO     (BaseReg->rCurrentTSO)
 #define SAVE_CurrentNursery (BaseReg->rCurrentNursery)
@@ -349,7 +348,7 @@ GLOBAL_REG_DECL(P_,Hp,REG_Hp)
 #endif
 
 #if defined(REG_HpLim) && !defined(NO_GLOBAL_REG_DECLS)
-GLOBAL_REG_DECL(P_,HpLim,REG_HpLim)
+#error HpLim cannot be in a register
 #else
 #define HpLim (BaseReg->rHpLim)
 #endif
@@ -570,14 +569,6 @@ GLOBAL_REG_DECL(bdescr *,HpAlloc,REG_HpAlloc)
 #define CALLER_RESTORE_Hp	/* nothing */
 #endif
 
-#ifdef CALLER_SAVES_HpLim
-#define CALLER_SAVE_HpLim   	SAVE_HpLim = HpLim;
-#define CALLER_RESTORE_HpLim	HpLim = SAVE_HpLim;
-#else
-#define CALLER_SAVE_HpLim   	/* nothing */
-#define CALLER_RESTORE_HpLim   	/* nothing */
-#endif
-
 #ifdef CALLER_SAVES_Base
 #ifdef THREADED_RTS
 #error "Can't have caller-saved BaseReg with THREADED_RTS"
@@ -644,7 +635,6 @@ GLOBAL_REG_DECL(bdescr *,HpAlloc,REG_HpAlloc)
   CALLER_SAVE_Sp				\
   CALLER_SAVE_SpLim				\
   CALLER_SAVE_Hp				\
-  CALLER_SAVE_HpLim				\
   CALLER_SAVE_CurrentTSO			\
   CALLER_SAVE_CurrentNursery			\
   CALLER_SAVE_Base
@@ -673,7 +663,6 @@ GLOBAL_REG_DECL(bdescr *,HpAlloc,REG_HpAlloc)
   CALLER_RESTORE_Sp				\
   CALLER_RESTORE_SpLim				\
   CALLER_RESTORE_Hp				\
-  CALLER_RESTORE_HpLim				\
   CALLER_RESTORE_CurrentTSO			\
   CALLER_RESTORE_CurrentNursery