1 Commits

Author	SHA1	Message	Date
Taewook Oh	0e35ea3b7c	Remove redundant copy in recurrences ... Summary: If there is a chain of instructions formulating a recurrence, commuting operands can help removing a redundant copy. In the following example code, ``` BB#1: ; Loop Header %vreg0<def> = COPY %vreg13<kill>; GR32:%vreg0,%vreg13 ... BB#6: ; Loop Latch %vreg2<def> = COPY %vreg15<kill>; GR32:%vreg2,%vreg15 %vreg10<def,tied1> = ADD32rr %vreg1<kill,tied0>, %vreg0<kill>, %EFLAGS<imp-def,dead>; GR32:%vreg10,%vreg1,%vreg0 %vreg3<def,tied1> = ADD32rr %vreg2<kill,tied0>, %vreg10<kill>, %EFLAGS<imp-def,dead>; GR32:%vreg3,%vreg2,%vreg10 CMP32ri8 %vreg3, 10, %EFLAGS<imp-def>; GR32:%vreg3 %vreg13<def> = COPY %vreg3<kill>; GR32:%vreg13,%vreg3 JL_1 <BB#1>, %EFLAGS<imp-use,kill> ``` Existing two-address generation pass generates following code: ``` BB#1: %vreg0<def> = COPY %vreg13<kill>; GR32:%vreg0,%vreg13 ... BB#6: Predecessors according to CFG: BB#5 BB#4 %vreg2<def> = COPY %vreg15<kill>; GR32:%vreg2,%vreg15 %vreg10<def> = COPY %vreg1<kill>; GR32:%vreg10,%vreg1 %vreg10<def,tied1> = ADD32rr %vreg10<tied0>, %vreg0<kill>, %EFLAGS<imp-def,dead>; GR32:%vreg10,%vreg0 %vreg3<def> = COPY %vreg10<kill>; GR32:%vreg3,%vreg10 %vreg3<def,tied1> = ADD32rr %vreg3<tied0>, %vreg2<kill>, %EFLAGS<imp-def,dead>; GR32:%vreg3,%vreg2 CMP32ri8 %vreg3, 10, %EFLAGS<imp-def>; GR32:%vreg3 %vreg13<def> = COPY %vreg3<kill>; GR32:%vreg13,%vreg3 JL_1 <BB#1>, %EFLAGS<imp-use,kill> JMP_1 <BB#7> ``` This is suboptimal because the assembly code generated has a redundant copy at the end of #BB6 to feed %vreg13 to BB#1: ``` .LBB0_6: addl %esi, %edi addl %ebx, %edi cmpl $10, %edi movl %edi, %esi jl .LBB0_1 ``` This redundant copy can be elimiated by making instructions in the recurrence chain to compute the value "into" the register that actually holds the feedback value. In this example, this can be achieved by commuting %vreg0 and %vreg1 to compute %vreg10. With that change, code after two-address generation becomes ``` BB#1: %vreg0<def> = COPY %vreg13<kill>; GR32:%vreg0,%vreg13 ... BB#6: derived from LLVM BB %bb7 Predecessors according to CFG: BB#5 BB#4 %vreg2<def> = COPY %vreg15<kill>; GR32:%vreg2,%vreg15 %vreg10<def> = COPY %vreg0<kill>; GR32:%vreg10,%vreg0 %vreg10<def,tied1> = ADD32rr %vreg10<tied0>, %vreg1<kill>, %EFLAGS<imp-def,dead>; GR32:%vreg10,%vreg1 %vreg3<def> = COPY %vreg10<kill>; GR32:%vreg3,%vreg10 %vreg3<def,tied1> = ADD32rr %vreg3<tied0>, %vreg2<kill>, %EFLAGS<imp-def,dead>; GR32:%vreg3,%vreg2 CMP32ri8 %vreg3, 10, %EFLAGS<imp-def>; GR32:%vreg3 %vreg13<def> = COPY %vreg3<kill>; GR32:%vreg13,%vreg3 JL_1 <BB#1>, %EFLAGS<imp-use,kill> JMP_1 <BB#7> ``` and the final assembly does not have redundant copy: ``` .LBB0_6: addl %edi, %eax addl %ebx, %eax cmpl $10, %eax jl .LBB0_1 ``` Reviewers: qcolombet, MatzeB, wmi Reviewed By: wmi Subscribers: llvm-commits Differential Revision: https://reviews.llvm.org/D31821 llvm-svn: 306758	2017-06-29 23:11:24 +00:00