curve25519: eliminate unnecessary "callee save" prologues

The assembly implementations of ladderstep and mul contain register
save prologues that are unnecessary in Go because there are no callee
save registers in the Go ABI. Remove these prologues, update all SP
offsets, and reduce the frame size accordingly.

The SP offsets were updated with:

python -c 'import sys, re; sys.stdout.write(re.sub(r"(\d+)\(SP\)", lambda m: "%d(SP)" % (int(m.group(1))-YYY), sys.stdin.read()))'

where YYY was 64 for mul_amd64.s and 56 for ladderstep_amd64.s.

Change-Id: I728948809f479b1c061cc65167dadad651efab31
Reviewed-on: https://go-review.googlesource.com/31580
Reviewed-by: Adam Langley <agl@golang.org>
Reviewed-by: Minux Ma <minux@golang.org>

curve25519/mul_amd64.s

@@ -8,29 +8,21 @@
 // +build amd64,!gccgo,!appengine
 
 // func mul(dest, a, b *[5]uint64)
-TEXT ·mul(SB),0,$80-24
+TEXT ·mul(SB),0,$16-24
 	MOVQ dest+0(FP), DI
 	MOVQ a+8(FP), SI
 	MOVQ b+16(FP), DX
 
-	MOVQ R11,0(SP)
-	MOVQ R12,8(SP)
-	MOVQ R13,16(SP)
-	MOVQ R14,24(SP)
-	MOVQ R15,32(SP)
-	MOVQ BX,40(SP)
-	MOVQ BP,48(SP)
-	MOVQ DI,56(SP)
 	MOVQ DX,CX
 	MOVQ 24(SI),DX
 	IMUL3Q $19,DX,AX
-	MOVQ AX,64(SP)
+	MOVQ AX,0(SP)
 	MULQ 16(CX)
 	MOVQ AX,R8
 	MOVQ DX,R9
 	MOVQ 32(SI),DX
 	IMUL3Q $19,DX,AX
-	MOVQ AX,72(SP)
+	MOVQ AX,8(SP)
 	MULQ 8(CX)
 	ADDQ AX,R8
 	ADCQ DX,R9
@@ -105,11 +97,11 @@ TEXT ·mul(SB),0,$80-24
 	MULQ 8(CX)
 	ADDQ AX,BX
 	ADCQ DX,BP
-	MOVQ 64(SP),AX
+	MOVQ 0(SP),AX
 	MULQ 24(CX)
 	ADDQ AX,R10
 	ADCQ DX,R11
-	MOVQ 64(SP),AX
+	MOVQ 0(SP),AX
 	MULQ 32(CX)
 	ADDQ AX,R12
 	ADCQ DX,R13
@@ -117,15 +109,15 @@ TEXT ·mul(SB),0,$80-24
 	MULQ 0(CX)
 	ADDQ AX,BX
 	ADCQ DX,BP
-	MOVQ 72(SP),AX
+	MOVQ 8(SP),AX
 	MULQ 16(CX)
 	ADDQ AX,R10
 	ADCQ DX,R11
-	MOVQ 72(SP),AX
+	MOVQ 8(SP),AX
 	MULQ 24(CX)
 	ADDQ AX,R12
 	ADCQ DX,R13
-	MOVQ 72(SP),AX
+	MOVQ 8(SP),AX
 	MULQ 32(CX)
 	ADDQ AX,R14
 	ADCQ DX,R15
@@ -172,13 +164,4 @@ TEXT ·mul(SB),0,$80-24
 	MOVQ R9,16(DI)
 	MOVQ AX,24(DI)
 	MOVQ R10,32(DI)
-	MOVQ 0(SP),R11
-	MOVQ 8(SP),R12
-	MOVQ 16(SP),R13
-	MOVQ 24(SP),R14
-	MOVQ 32(SP),R15
-	MOVQ 40(SP),BX
-	MOVQ 48(SP),BP
-	MOVQ DI,AX
-	MOVQ SI,DX
 	RET