[PATCH] D129352: [CodeGen] Limit building time for CodeGenPrepare

[Xiang Zhang via Phabricator via llvm-commits]

xiangzhangllvm added inline comments.


================
Comment at: llvm/lib/CodeGen/CodeGenPrepare.cpp:7103
+      // So we put the operands defs' BBs into FreshBBs to do optmization.
+      for (unsigned I = 0; I < NI->getNumOperands(); ++I) {
+        auto *OpDef = dyn_cast<Instruction>(NI->getOperand(I));
----------------
LuoYuanke wrote:
> xiangzhangllvm wrote:
> > xiangzhangllvm wrote:
> > > LuoYuanke wrote:
> > > > xiangzhangllvm wrote:
> > > > > LuoYuanke wrote:
> > > > > > The instruction is cloned. Why the def of its operand need to be sinked?
> > > > > When we copy an Instruction (to a new place) to do optimization, means we updated an instruction. This instruction's operand defs may have new opportunity to sink to the optimized new instruction.
> > > > Here we just copy the instruction. We know it is sinked when it is erased in line 7135.
> > > Let me try give an example soon.
> > I find an ARM case about it, it not about sink, it about copy 2 times:
> > 
> > **1st time**: copy insertelement from entry to vector.body
> > ```
> > entry:
> >   ...
> > 1  %l0 = trunc i16 %x to i8
> > 2  %l1 = insertelement <8 x i8> undef, i8 %l0, i32 0
> >   ...
> > 3  br label %vector.body
> > 
> > vector.body:                                      ; preds = %vector.body, %entry
> >   ...
> > 4  %0 = insertelement <8 x i8> undef, i8 %l0, i32 0       // copy from line 2
> > 5  %1 = shufflevector <8 x i8> %0, <8 x i8> undef, <8 x i32> zeroinitializer
> > 6  %l9 = mul <8 x i8> %1, %l8
> > ```
> > 
> > **2nd time**, copy insertelement's operand trunc
> > 
> > ```
> >   ...
> > 1  %l0 = trunc i16 %x to i8 
> > 2  %l1 = insertelement <8 x i8> undef, i8 %l0, i32 0
> >   ...
> > 3  br label %vector.body
> > 
> > vector.body:                                      ; preds = %vector.body, %entry
> >   ...
> >   %0 = trunc i16 %x to i8                           // 2nd copy optimization
> > 
> >   %1 = insertelement <8 x i8> undef, i8 %0, i32 0    // use 2nd copy
> >   %2 = shufflevector <8 x i8> %1, <8 x i8> undef, <8 x i32> zeroinitializer
> >   %l9 = mul <8 x i8> %2, %l8
> > 
> > ```
> Is the `insertelement` in entry BB erased for the first time?
It is not erased, because it still has user in entry BB.
But it is possible to erase.


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  https://reviews.llvm.org/D129352/new/

https://reviews.llvm.org/D129352