mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2025-01-31 12:41:49 +01:00
7e3c6fd16c
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
Low Level Virtual Machine (LLVM) ================================ This directory and its subdirectories contain source code for LLVM, a toolkit for the construction of highly optimized compilers, optimizers, and runtime environments. LLVM is open source software. You may freely distribute it under the terms of the license agreement found in LICENSE.txt. Please see the documentation provided in docs/ for further assistance with LLVM, and in particular docs/GettingStarted.rst for getting started with LLVM and docs/README.txt for an overview of LLVM's documentation setup. If you are writing a package for LLVM, see docs/Packaging.rst for our suggestions.