[PATCH] D97479: BPF: Implement TTI.getCmpSelInstrCost() properly

[Yonghong Song via Phabricator via llvm-commits]

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The Select insn in BPF is expensive as BPF backend
needs to resolve with conditionals. Similarly to
SystemZ, if this happens, return the cost of 4
instead of default 1.

This change is motivated during bcc code review for

  https://github.com/iovisor/bcc/pull/3270

where IndVarSimplifyPass eventually caused generating
the following asm code:

  ;       for (i = 0; (i < VIRTIO_MAX_SGS) && (i < num); i++) {
      14:       16 05 40 00 00 00 00 00 if w5 == 0 goto +64 <LBB0_6>
      15:       bc 51 00 00 00 00 00 00 w1 = w5
      16:       04 01 00 00 ff ff ff ff w1 += -1
      17:       67 05 00 00 20 00 00 00 r5 <<= 32
      18:       77 05 00 00 20 00 00 00 r5 >>= 32
      19:       a6 01 01 00 05 00 00 00 if w1 < 5 goto +1 <LBB0_4>
      20:       b7 05 00 00 06 00 00 00 r5 = 6 
  00000000000000a8 <LBB0_4>:
      21:       b7 02 00 00 00 00 00 00 r2 = 0 
      22:       b7 01 00 00 00 00 00 00 r1 = 0 
  ;       for (i = 0; (i < VIRTIO_MAX_SGS) && (i < num); i++) {
      23:       7b 1a e0 ff 00 00 00 00 *(u64 *)(r10 - 32) = r1
      24:       7b 5a c0 ff 00 00 00 00 *(u64 *)(r10 - 64) = r5

Note that insn #15 has w1 = w5 and w1 is refined later but r5(w5) is
eventually saved on stack at insn #24 for later use. This cause
later verifier failures.

With this change which introduced proper cost model
for Select instruction, IndVarSimplifyPass won't do
the above transformation any more.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D97479

Files:
  llvm/lib/Target/BPF/BPFTargetTransformInfo.h


Index: llvm/lib/Target/BPF/BPFTargetTransformInfo.h
===================================================================
--- llvm/lib/Target/BPF/BPFTargetTransformInfo.h
+++ llvm/lib/Target/BPF/BPFTargetTransformInfo.h
@@ -42,6 +42,17 @@
 
     return TTI::TCC_Basic;
   }
+
+  int getCmpSelInstrCost(unsigned Opcode, Type *ValTy, Type *CondTy,
+                         CmpInst::Predicate VecPred,
+                         TTI::TargetCostKind CostKind,
+                         const llvm::Instruction *I = nullptr) {
+    if (Opcode == Instruction::Select)
+      return 4;
+
+    return BaseT::getCmpSelInstrCost(Opcode, ValTy, CondTy, VecPred, CostKind,
+                                     I);
+  }
 };
 
 } // end namespace llvm


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