[PATCH] D127115: [RFC][DAGCombine] Make sure combined nodes are added back to the worklist in topological order.

[Amaury SECHET via Phabricator via llvm-commits]

deadalnix added inline comments.


================
Comment at: llvm/test/CodeGen/PowerPC/testComparesigeuc.ll:72
+; CHECK-NEXT:    not r3, r3
+; CHECK-NEXT:    clrlwi r3, r3, 31
 ; CHECK-NEXT:    stb r3, glob at toc@l(r5)
----------------
Both start from:
```
SelectionDAG has 28 nodes:
  t0: ch = EntryToken
      t2: i64,ch = CopyFromReg t0, Register:i64 %0
    t28: i64 = AssertZext t2, ValueType:ch:i8
  t31: i32 = truncate t28
      t7: i64,ch = CopyFromReg t0, Register:i64 %1
    t25: i64 = AssertZext t7, ValueType:ch:i8
  t32: i32 = truncate t25
  t16: i1 = setcc t31, t32, setuge:ch
                t46: i64 = zero_extend t31, Constant:i32<64>
                t47: i64 = zero_extend t32, Constant:i32<64>
              t48: i64 = sub t46, t47
            t50: i64 = srl t48, Constant:i32<63>
          t52: i64 = xor t50, Constant:i64<1>
        t53: i1 = truncate t52
      t36: i32 = zero_extend t53
      t42: i64,ch = PPCISD::TOC_ENTRY<(load (s64) from got)> TargetGlobalAddress:i64<i8* @glob> 0,$
    t37: ch = store<(store (s8) into @glob), trunc to i8> t0, t36, t42, undef:i64
  t22: ch = PPCISD::RET_FLAG t37
```

With this patch, the first significant node to be combined is `t53: i1 = truncate t52`, which goes through SimplifytDemandedBits and the DAG comes out as:
```
SelectionDAG has 28 nodes:
  t0: ch = EntryToken
            t2: i64,ch = CopyFromReg t0, Register:i64 %0
          t28: i64 = AssertZext t2, ValueType:ch:i8
        t31: i32 = truncate t28
      t46: i64 = zero_extend t31, Constant:i32<64>
            t7: i64,ch = CopyFromReg t0, Register:i64 %1
          t25: i64 = AssertZext t7, ValueType:ch:i8
        t32: i32 = truncate t25
      t47: i64 = zero_extend t32, Constant:i32<64>
    t48: i64 = sub t46, t47
  t50: i64 = srl t48, Constant:i32<63>
  t52: i64 = xor t50, Constant:i64<1>
          t55: i64 = xor t50, Constant:i64<-1>
        t53: i1 = truncate t55
      t36: i32 = zero_extend t53
      t42: i64,ch = PPCISD::TOC_ENTRY<(load (s64) from got)> TargetGlobalAddress:i64<i8* @glob> 0,$
    t37: ch = store<(store (s8) into @glob), trunc to i8> t0, t36, t42, undef:i64
  t22: ch = PPCISD::RET_FLAG t37
```

Previously, the first significant node to be combined was `t52: i64 = xor t50, Constant:i64<1>`, creating this DAG:
```
SelectionDAG has 28 nodes:
  t0: ch = EntryToken
          t2: i64,ch = CopyFromReg t0, Register:i64 %0
        t28: i64 = AssertZext t2, ValueType:ch:i8
      t31: i32 = truncate t28
    t46: i64 = zero_extend t31, Constant:i32<64>
          t7: i64,ch = CopyFromReg t0, Register:i64 %1
        t25: i64 = AssertZext t7, ValueType:ch:i8
      t32: i32 = truncate t25
    t47: i64 = zero_extend t32, Constant:i32<64>
  t48: i64 = sub t46, t47
    t50: i64 = srl t48, Constant:i32<63>
  t52: i64 = xor t50, Constant:i64<1>
          t56: i64 = xor t48, Constant:i64<-1>
        t57: i64 = srl t56, Constant:i32<63>
      t54: i32 = truncate t57
      t42: i64,ch = PPCISD::TOC_ENTRY<(load (s64) from got)> TargetGlobalAddress:i64<i8* @glob> 0, $
    t37: ch = store<(store (s8) into @glob), trunc to i8> t0, t54, t42, undef:i64
  t22: ch = PPCISD::RET_FLAG t37
```

Which creates a ton redundancy, but that gets cleaned up.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D127115/new/

https://reviews.llvm.org/D127115