mips64: Don't store REGTMP with GPRS

REGTMP is the scratch register used by the assembler. Saving it with
GPRS is bad because one needs to be extra careful writing assembly that
won't clobber REGTMP as long as it's not saved or after it's restored.

Instead try to save it as early and restore it as late as possible to
prevent bugs, that will only be visible in the disassembly.

Author: clktmr

src/runtime/tasker_noos_mips64.go

@@ -24,15 +24,15 @@ import (
 
 // See saveGPRS and saveFPRS
 const (
-	numGPRS = 28
+	numGPRS = 27
 	numFPRS = 33
 )
 
 type mOS struct {
 	// thread context
-	gprs            [numGPRS]uintptr
-	fprs            [numFPRS]float64
-	sp, fp, ra, epc uintptr
+	gprs                 [numGPRS]uintptr
+	fprs                 [numFPRS]float64
+	sp, fp, ra, epc, tmp uintptr
 }
 
 var (

src/runtime/tasker_noos_mips64.s

@@ -46,7 +46,8 @@
 #define _lr        (0*8)
 #define _mstatus   (1*8)
 #define _mepc      (2*8)
-#define excCtxSize (3*8)
+#define _mregtmp   (3*8)
+#define excCtxSize (4*8)
 
 
 // This will be copied into the processor's general exception vector.  Since the
@@ -69,6 +70,9 @@ TEXT runtime·intvector(SB),NOSPLIT|NOFRAME,$0
 // Only syscalls and interrupts are handled at the moment, all other exceptions
 // are fatal.
 TEXT runtime·exceptionHandler(SB),NOSPLIT|NOFRAME,$0
+	// Be especially careful to not clobber REGTMP with multi-instruction
+	// statements until it's saved on the stack.
+
 	// Determine caller stack
 	MOVV  $·cpu0(SB), R26
 	BNE   R26, g, fromThread
@@ -89,6 +93,7 @@ fromThread:
 fromHandler:
 	// Save exception context on ISR stack
 	SUB   $excCtxSize, R29
+	MOVV  R23, _mregtmp(R29) // R23 (REGTMP) is now free for use
 	OR    $1, R31, R26 // Encode smallCtx flag in lr
 	MOVV  R26, _lr(R29) // R29 is now free for use
 	MOVV  M(C0_SR), R26
@@ -170,7 +175,8 @@ TEXT runtime·syscallHandler(SB),NOSPLIT|NOFRAME,$0
 
 	MOVV  _lr(R29), R3
 	MOVV  R3, (m_mOS+mOS_ra)(R2)
-
+	MOVV  _mregtmp(R29), R3
+	MOVV  R3, (m_mOS+mOS_tmp)(R2)
 	MOVV  _mepc(R29), R3
 	AND   $~1, R3  // Remove fromHandler flag from epc
 	MOVV  R3, (m_mOS+mOS_epc)(R2)
@@ -249,6 +255,9 @@ TEXT runtime·softwareInterruptHandler(SB),NOSPLIT|NOFRAME,$0
 	// Save thread context in mOS
 	MOVV  (cpuctx_exe)(g), R27
 
+	MOVV  _mregtmp(R29), R26
+	MOVV  R26, (m_mOS+mOS_tmp)(R27)
+
 	MOVV  _lr(R29), R26
 	AND   $~1, R26  // Remove smallCtx flag from lr
 	MOVV  R26, (m_mOS+mOS_ra)(R27)
@@ -315,8 +324,8 @@ smallCtx:
 	MOVV  (m_mOS+mOS_ra)(R27), R31
 	MOVV  (m_mOS+mOS_epc)(R27), R26
 	MOVV  R26, M(C0_EPC)
-	MOVV  $~1, R27
-	AND   R27, R31 // Remove smallCtx flag
+	AND   $~1, R31 // Remove smallCtx flag
+	MOVV  (m_mOS+mOS_tmp)(R27), R23
 
 	ERET
 
@@ -426,11 +435,9 @@ TEXT runtime·exceptionReturn(SB),NOSPLIT|NOFRAME,$0
 	MOVV  $1, R27
 	AND   R26, R27
 
-	ADD   $excCtxSize, R29
-
 	// Don't restore interrupt mask or switch stacks yet if we were called
 	// from handler
-	BNE   R27, R0, return
+	BNE   R27, R0, fromHandler
 
 	MOVW  M(C0_SR), R26
 	MOVW  $~INTR_EXT, R27
@@ -441,11 +448,18 @@ TEXT runtime·exceptionReturn(SB),NOSPLIT|NOFRAME,$0
 	OR    R27, R26
 	MOVW  R26, M(C0_SR)
 
+	MOVV  _mregtmp(R29), R23
+	ADD   $excCtxSize, R29
 	MOVV  $·cpu0(SB), R26
 	MOVV  (g_sched+gobuf_sp)(R26), R29
 	MOVV  (g_sched+gobuf_g)(R26), g
 
-return:
+	ERET
+
+fromHandler:
+	MOVV  _mregtmp(R29), R23
+	ADD   $excCtxSize, R29
+
 	ERET
 
 
@@ -478,29 +492,25 @@ TEXT ·saveGPRs(SB),NOSPLIT|NOFRAME,$0
 	MOVV R20, 152(R26)
 	MOVV R21, 160(R26)
 	MOVV R22, 168(R26)
-	MOVV R23, 176(R26)
-	MOVV R24, 184(R26)
-	MOVV R25, 192(R26)
-	MOVV RSB, 200(R26)
+	MOVV R24, 176(R26)
+	MOVV R25, 184(R26)
+	MOVV RSB, 192(R26)
 	MOVV HI, R1
-	MOVV R1, 208(R26)
+	MOVV R1, 200(R26)
 	MOVV LO, R1
-	MOVV R1, 216(R26)
+	MOVV R1, 208(R26)
 	RET
 
 
-// R26 must point to stored gprs.  Only use R26, R27 after restoring.  Be
-// especially careful and look at the disassembly;  The assembler might decide
-// to use R16-R23 for you.
+// R26 must point to stored gprs.  Only use R26, R27 after restoring.
 TEXT ·restoreGPRs(SB),NOSPLIT|NOFRAME,$0
-	MOVV 216(R26), R1
-	MOVV R1, LO
 	MOVV 208(R26), R1
+	MOVV R1, LO
+	MOVV 200(R26), R1
 	MOVV R1, HI
-	MOVV 200(R26), RSB
-	MOVV 192(R26), R25
-	MOVV 184(R26), R24
-	MOVV 176(R26), R23
+	MOVV 192(R26), RSB
+	MOVV 184(R26), R25
+	MOVV 176(R26), R24
 	MOVV 168(R26), R22
 	MOVV 160(R26), R21
 	MOVV 152(R26), R20