Hi, all. I find the benefits of this patch come from changing a 64bit MUL
(3 32bit MUL and 2 32bit ADD) to a 32bit MUL. At present, the
optimization is done as a magic replacement, which is not a common way to
generate code. (https://issues.apache.org/jira/browse/HARMONY5826)
Assume A and B are both 64bit operands and we are doing A*B. In 32bit
machine, the MUL operation is usually translated to (High 32bit of A)*(Low
32bit of b)+(Low 32bit of A)*(High 32bit of B)+(Low 32bit of A)*(Low
32bit of A). But when we know High 32bit of A and B are both 0, only (Low
32bit of A)*(Low 32bit of A) needed.
Following are the HIR and LIR for result = (a & ffffffffL) * (b & ffffffffL)
+ (c & ffffffffL) + (d & ffffffffL) without this patch. We can do the
optimization in HIR simplifier or LIR peephole. I'm not sure whether
changing int64 operation to int32 operation will bring overhead for 64bit
machine. I think maybe peephole is a better place. I find there is no
peephole optimization for XOR. If JIT could find out the result of XOR is 0,
then propagates the 0 to MUL and related MUL is eliminated. The left problem
is whether there is sufficient dataflow analysis in LIR to do the
propagation and elimination.
=====HIR=====
I42:ldci8 #4294967295 ) t38:int64
I43:and t37, t38 ) t39:int64
I44:convi8 g23 ) t40:int64
I45:and t40, t38 ) t41:int64
I46:mul t39, t41 ) t42:int64
=====LIR=====
238B02A6 I329: MOV s286[v208(ESP)+t285(100)]:I_32,v19(EAX):I_32
238B02AA I328: MOV t291[v208(ESP)+t290(104)]:I_32,v19(EAX):I_32
238B02AE I327: (ID:s8(EFLGS):U_32) =XOR t206(EDX):I_32,t206(EDX):I_32
238B02B0 I326: MOV s292(EAX):I_32,s286[v208(ESP)+t285(100)]:I_32
238B02B4 I70: (ID:s8(EFLGS):U_32) =MUL
s139(EDX):I_32,s292(EAX):I_32,t206(EDX):I_32
238B02B6 I325: MOV s140(EBX):I_32,s292(EAX):I_32
238B02B8 I324: (ID:s8(EFLGS):U_32) =XOR s292(EAX):I_32,s292(EAX):I_32
238B02BA I323: MOV t289(EDI):I_32,v217[v208(ESP)+t216(24)]:I_32
238B02BE I73: (ID:s8(EFLGS):U_32) =MUL
s139(EDX):I_32,s292(EAX):I_32,t289(EDI):I_32
238B02C0 I74: (ID:s8(EFLGS):U_32) =ADD s140(EBX):I_32,s292(EAX):I_32
238B02C2 I322: MOV v19(EAX):I_32,t291[v208(ESP)+t290(104)]:I_32
DEADBEEF I75: (AD:s293(EAX):I_32) =CopyPseudoInst/MOV (AU:v19(EAX):I_32)
238B02C6 I76: (ID:s8(EFLGS):U_32) =MUL
s139(EDX):I_32,s293(EAX):I_32,t289(EDI):I_32
238B02C8 I321: MOV s286[v208(ESP)+t285(100)]:I_32,s293(EAX):I_32
238B02CC I77: (ID:s8(EFLGS):U_32) =ADD s139(EDX):I_32,s140(EBX):I_32
Any comments? Thanks.
Xiaoming
