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sync/atomic: replace MFENCE with LOCK XADD
MFENCE was introduced only on the Pentium4 (SSE2), while XADD was introduced on the 486. Fixes #2268. R=golang-dev, rsc CC=fshahriar, golang-dev https://golang.org/cl/5056045
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@ -103,6 +103,7 @@ TEXT ·LoadInt64(SB),7,$0
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TEXT ·LoadUint64(SB),7,$0
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MOVL addrptr+0(FP), AX
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// MOVQ and EMMS were introduced on the Pentium MMX.
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// MOVQ (%EAX), %MM0
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BYTE $0x0f; BYTE $0x6f; BYTE $0x00
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// MOVQ %MM0, 0x8(%ESP)
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@ -131,14 +132,18 @@ TEXT ·StoreInt64(SB),7,$0
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TEXT ·StoreUint64(SB),7,$0
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MOVL addrptr+0(FP), AX
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// MOVQ and EMMS were introduced on the Pentium MMX.
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// MOVQ 0x8(%ESP), %MM0
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BYTE $0x0f; BYTE $0x6f; BYTE $0x44; BYTE $0x24; BYTE $0x08
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// MOVQ %MM0, (%EAX)
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BYTE $0x0f; BYTE $0x7f; BYTE $0x00
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// EMMS
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BYTE $0x0F; BYTE $0x77
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// MFENCE
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BYTE $0x0f; BYTE $0xae; BYTE $0xf0
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// This is essentially a no-op, but it provides required memory fencing.
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// It can be replaced with MFENCE, but MFENCE was introduced only on the Pentium4 (SSE2).
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XORL AX, AX
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LOCK
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XADDL AX, (SP)
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RET
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TEXT ·StoreUintptr(SB),7,$0
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