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This patch introduces a new pass that computes the safe point to insert the
prologue and epilogue of the function.
The interest is to find safe points that are cheaper than the entry and exits
blocks.
As an example and to avoid regressions to be introduce, this patch also
implements the required bits to enable the shrink-wrapping pass for AArch64.
** Context **
Currently we insert the prologue and epilogue of the method/function in the
entry and exits blocks. Although this is correct, we can do a better job when
those are not immediately required and insert them at less frequently executed
places.
The job of the shrink-wrapping pass is to identify such places.
** Motivating example **
Let us consider the following function that perform a call only in one branch of
a if:
define i32 @f(i32 %a, i32 %b) {
%tmp = alloca i32, align 4
%tmp2 = icmp slt i32 %a, %b
br i1 %tmp2, label %true, label %false
true:
store i32 %a, i32* %tmp, align 4
%tmp4 = call i32 @doSomething(i32 0, i32* %tmp)
br label %false
false:
%tmp.0 = phi i32 [ %tmp4, %true ], [ %a, %0 ]
ret i32 %tmp.0
}
On AArch64 this code generates (removing the cfi directives to ease
readabilities):
_f: ; @f
; BB#0:
stp x29, x30, [sp, #-16]!
mov x29, sp
sub sp, sp, #16 ; =16
cmp w0, w1
b.ge LBB0_2
; BB#1: ; %true
stur w0, [x29, #-4]
sub x1, x29, #4 ; =4
mov w0, wzr
bl _doSomething
LBB0_2: ; %false
mov sp, x29
ldp x29, x30, [sp], #16
ret
With shrink-wrapping we could generate:
_f: ; @f
; BB#0:
cmp w0, w1
b.ge LBB0_2
; BB#1: ; %true
stp x29, x30, [sp, #-16]!
mov x29, sp
sub sp, sp, #16 ; =16
stur w0, [x29, #-4]
sub x1, x29, #4 ; =4
mov w0, wzr
bl _doSomething
add sp, x29, #16 ; =16
ldp x29, x30, [sp], #16
LBB0_2: ; %false
ret
Therefore, we would pay the overhead of setting up/destroying the frame only if
we actually do the call.
** Proposed Solution **
This patch introduces a new machine pass that perform the shrink-wrapping
analysis (See the comments at the beginning of ShrinkWrap.cpp for more details).
It then stores the safe save and restore point into the MachineFrameInfo
attached to the MachineFunction.
This information is then used by the PrologEpilogInserter (PEI) to place the
related code at the right place. This pass runs right before the PEI.
Unlike the original paper of Chow from PLDI’88, this implementation of
shrink-wrapping does not use expensive data-flow analysis and does not need hack
to properly avoid frequently executed point. Instead, it relies on dominance and
loop properties.
The pass is off by default and each target can opt-in by setting the
EnableShrinkWrap boolean to true in their derived class of TargetPassConfig.
This setting can also be overwritten on the command line by using
-enable-shrink-wrap.
Before you try out the pass for your target, make sure you properly fix your
emitProlog/emitEpilog/adjustForXXX method to cope with basic blocks that are not
necessarily the entry block.
** Design Decisions **
1. ShrinkWrap is its own pass right now. It could frankly be merged into PEI but
for debugging and clarity I thought it was best to have its own file.
2. Right now, we only support one save point and one restore point. At some
point we can expand this to several save point and restore point, the impacted
component would then be:
- The pass itself: New algorithm needed.
- MachineFrameInfo: Hold a list or set of Save/Restore point instead of one
pointer.
- PEI: Should loop over the save point and restore point.
Anyhow, at least for this first iteration, I do not believe this is interesting
to support the complex cases. We should revisit that when we motivating
examples.
Differential Revision: http://reviews.llvm.org/D9210
<rdar://problem/3201744>
llvm-svn: 236507
|
||
|---|---|---|
| .. | ||
| AsmParser | ||
| Disassembler | ||
| InstPrinter | ||
| MCTargetDesc | ||
| TargetInfo | ||
| CMakeLists.txt | ||
| DelaySlotFiller.cpp | ||
| LLVMBuild.txt | ||
| Makefile | ||
| README.txt | ||
| Sparc.h | ||
| Sparc.td | ||
| SparcAsmPrinter.cpp | ||
| SparcCallingConv.td | ||
| SparcFrameLowering.cpp | ||
| SparcFrameLowering.h | ||
| SparcInstr64Bit.td | ||
| SparcInstrAliases.td | ||
| SparcInstrFormats.td | ||
| SparcInstrInfo.cpp | ||
| SparcInstrInfo.h | ||
| SparcInstrInfo.td | ||
| SparcInstrVIS.td | ||
| SparcISelDAGToDAG.cpp | ||
| SparcISelLowering.cpp | ||
| SparcISelLowering.h | ||
| SparcMachineFunctionInfo.cpp | ||
| SparcMachineFunctionInfo.h | ||
| SparcMCInstLower.cpp | ||
| SparcRegisterInfo.cpp | ||
| SparcRegisterInfo.h | ||
| SparcRegisterInfo.td | ||
| SparcSelectionDAGInfo.cpp | ||
| SparcSelectionDAGInfo.h | ||
| SparcSubtarget.cpp | ||
| SparcSubtarget.h | ||
| SparcTargetMachine.cpp | ||
| SparcTargetMachine.h | ||
| SparcTargetObjectFile.cpp | ||
| SparcTargetObjectFile.h | ||
| SparcTargetStreamer.h | ||
To-do
-----
* Keep the address of the constant pool in a register instead of forming its
address all of the time.
* We can fold small constant offsets into the %hi/%lo references to constant
pool addresses as well.
* When in V9 mode, register allocate %icc[0-3].
* Add support for isel'ing UMUL_LOHI instead of marking it as Expand.
* Emit the 'Branch on Integer Register with Prediction' instructions. It's
not clear how to write a pattern for this though:
float %t1(int %a, int* %p) {
%C = seteq int %a, 0
br bool %C, label %T, label %F
T:
store int 123, int* %p
br label %F
F:
ret float undef
}
codegens to this:
t1:
save -96, %o6, %o6
1) subcc %i0, 0, %l0
1) bne .LBBt1_2 ! F
nop
.LBBt1_1: ! T
or %g0, 123, %l0
st %l0, [%i1]
.LBBt1_2: ! F
restore %g0, %g0, %g0
retl
nop
1) should be replaced with a brz in V9 mode.
* Same as above, but emit conditional move on register zero (p192) in V9
mode. Testcase:
int %t1(int %a, int %b) {
%C = seteq int %a, 0
%D = select bool %C, int %a, int %b
ret int %D
}
* Emit MULX/[SU]DIVX instructions in V9 mode instead of fiddling
with the Y register, if they are faster.
* Codegen bswap(load)/store(bswap) -> load/store ASI
* Implement frame pointer elimination, e.g. eliminate save/restore for
leaf fns.
* Fill delay slots
* Use %g0 directly to materialize 0. No instruction is required.