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

[LuoYuanke via Phabricator via llvm-commits]

LuoYuanke 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));
----------------
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?


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

https://reviews.llvm.org/D129352