[release-branch.go1.14] cmd/compile, runtime: mark R12 clobbered for write barrier call on PPC64

When external linking, for large binaries, the external linker
may insert a trampoline for the write barrier call, which looks

0000000005a98cc8 <__long_branch_runtime.gcWriteBarrier>:
 5a98cc8:       86 01 82 3d     addis   r12,r2,390
 5a98ccc:       d8 bd 8c e9     ld      r12,-16936(r12)
 5a98cd0:       a6 03 89 7d     mtctr   r12
 5a98cd4:       20 04 80 4e     bctr

It clobbers R12 (and CTR, which is never live across a call).

As at compile time we don't know whether the binary is big and
what link mode will be used, I think we need to mark R12 as
clobbered for write barrier call. For extra safety (future-proof)
we mark caller-saved register that cannot be used for function
arguments, which includes R11, as potentially clobbered as well.

Updates #40851.
Fixes #40938.

Change-Id: Iedd901c5072f1127cc59b0a48cfeb4aaec81b519
Reviewed-on: https://go-review.googlesource.com/c/go/+/248917
Run-TryBot: Cherry Zhang <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Austin Clements <[email protected]>
(cherry picked from commit b58d297)
Reviewed-on: https://go-review.googlesource.com/c/go/+/249697

Author: cherrymui

src/cmd/compile/internal/ssa/gen/PPC64Ops.go

@@ -561,9 +561,9 @@ func init() {
 		{name: "LoweredAtomicOr8", argLength: 3, reg: gpstore, asm: "OR", faultOnNilArg0: true, hasSideEffects: true},
 
 		// LoweredWB invokes runtime.gcWriteBarrier. arg0=destptr, arg1=srcptr, arg2=mem, aux=runtime.gcWriteBarrier
-		// It preserves R0 through R15, g, and its arguments R20 and R21,
+		// It preserves R0 through R17 (except special registers R1, R2, R11, R12, R13), g, and its arguments R20 and R21,
 		// but may clobber anything else, including R31 (REGTMP).
-		{name: "LoweredWB", argLength: 3, reg: regInfo{inputs: []regMask{buildReg("R20"), buildReg("R21")}, clobbers: (callerSave &^ buildReg("R0 R3 R4 R5 R6 R7 R8 R9 R10 R11 R12 R14 R15 R20 R21 g")) | buildReg("R31")}, clobberFlags: true, aux: "Sym", symEffect: "None"},
+		{name: "LoweredWB", argLength: 3, reg: regInfo{inputs: []regMask{buildReg("R20"), buildReg("R21")}, clobbers: (callerSave &^ buildReg("R0 R3 R4 R5 R6 R7 R8 R9 R10 R14 R15 R16 R17 R20 R21 g")) | buildReg("R31")}, clobberFlags: true, aux: "Sym", symEffect: "None"},
 
 		// There are three of these functions so that they can have three different register inputs.
 		// When we check 0 <= c <= cap (A), then 0 <= b <= c (B), then 0 <= a <= b (C), we want the

src/cmd/compile/internal/ssa/opGen.go

@@ -916,23 +916,23 @@ TEXT ·checkASM(SB),NOSPLIT,$0-1
 // - R20 is the destination of the write
 // - R21 is the value being written at R20.
 // It clobbers condition codes.
-// It does not clobber R0 through R15,
+// It does not clobber R0 through R17 (except special registers),
 // but may clobber any other register, *including* R31.
 TEXT runtime·gcWriteBarrier(SB),NOSPLIT,$112
 	// The standard prologue clobbers R31.
-	// We use R16 and R17 as scratch registers.
-	MOVD	g_m(g), R16
-	MOVD	m_p(R16), R16
-	MOVD	(p_wbBuf+wbBuf_next)(R16), R17
+	// We use R18 and R19 as scratch registers.
+	MOVD	g_m(g), R18
+	MOVD	m_p(R18), R18
+	MOVD	(p_wbBuf+wbBuf_next)(R18), R19
 	// Increment wbBuf.next position.
-	ADD	$16, R17
-	MOVD	R17, (p_wbBuf+wbBuf_next)(R16)
-	MOVD	(p_wbBuf+wbBuf_end)(R16), R16
-	CMP	R16, R17
+	ADD	$16, R19
+	MOVD	R19, (p_wbBuf+wbBuf_next)(R18)
+	MOVD	(p_wbBuf+wbBuf_end)(R18), R18
+	CMP	R18, R19
 	// Record the write.
-	MOVD	R21, -16(R17)	// Record value
-	MOVD	(R20), R16	// TODO: This turns bad writes into bad reads.
-	MOVD	R16, -8(R17)	// Record *slot
+	MOVD	R21, -16(R19)	// Record value
+	MOVD	(R20), R18	// TODO: This turns bad writes into bad reads.
+	MOVD	R18, -8(R19)	// Record *slot
 	// Is the buffer full? (flags set in CMP above)
 	BEQ	flush
 ret:
@@ -956,11 +956,12 @@ flush:
 	MOVD	R8, (FIXED_FRAME+56)(R1)
 	MOVD	R9, (FIXED_FRAME+64)(R1)
 	MOVD	R10, (FIXED_FRAME+72)(R1)
-	MOVD	R11, (FIXED_FRAME+80)(R1)
-	MOVD	R12, (FIXED_FRAME+88)(R1)
+	// R11, R12 may be clobbered by external-linker-inserted trampoline
 	// R13 is REGTLS
-	MOVD	R14, (FIXED_FRAME+96)(R1)
-	MOVD	R15, (FIXED_FRAME+104)(R1)
+	MOVD	R14, (FIXED_FRAME+80)(R1)
+	MOVD	R15, (FIXED_FRAME+88)(R1)
+	MOVD	R16, (FIXED_FRAME+96)(R1)
+	MOVD	R17, (FIXED_FRAME+104)(R1)
 
 	// This takes arguments R20 and R21.
 	CALL	runtime·wbBufFlush(SB)
@@ -975,10 +976,10 @@ flush:
 	MOVD	(FIXED_FRAME+56)(R1), R8
 	MOVD	(FIXED_FRAME+64)(R1), R9
 	MOVD	(FIXED_FRAME+72)(R1), R10
-	MOVD	(FIXED_FRAME+80)(R1), R11
-	MOVD	(FIXED_FRAME+88)(R1), R12
-	MOVD	(FIXED_FRAME+96)(R1), R14
-	MOVD	(FIXED_FRAME+104)(R1), R15
+	MOVD	(FIXED_FRAME+80)(R1), R14
+	MOVD	(FIXED_FRAME+88)(R1), R15
+	MOVD	(FIXED_FRAME+96)(R1), R16
+	MOVD	(FIXED_FRAME+104)(R1), R17
 	JMP	ret
 
 // Note: these functions use a special calling convention to save generated code space.