[llvm] [InstCombine] Try the flipped strictness of predicate in `foldICmpShlConstant` (PR #92773)

[Yingwei Zheng via llvm-commits]

================
@@ -2414,14 +2414,37 @@ Instruction *InstCombinerImpl::foldICmpShlConstant(ICmpInst &Cmp,
   // free on the target. It has the additional benefit of comparing to a
   // smaller constant that may be more target-friendly.
   unsigned Amt = ShiftAmt->getLimitedValue(TypeBits - 1);
-  if (Shl->hasOneUse() && Amt != 0 && C.countr_zero() >= Amt &&
-      DL.isLegalInteger(TypeBits - Amt)) {
-    Type *TruncTy = IntegerType::get(Cmp.getContext(), TypeBits - Amt);
-    if (auto *ShVTy = dyn_cast<VectorType>(ShType))
-      TruncTy = VectorType::get(TruncTy, ShVTy->getElementCount());
-    Constant *NewC =
-        ConstantInt::get(TruncTy, C.ashr(*ShiftAmt).trunc(TypeBits - Amt));
-    return new ICmpInst(Pred, Builder.CreateTrunc(X, TruncTy), NewC);
+  if (Shl->hasOneUse() && Amt != 0 && DL.isLegalInteger(TypeBits - Amt)) {
+    auto FoldICmpShlToICmpTrunc = [&](ICmpInst::Predicate Pred,
+                                      const APInt &C) -> Instruction * {
+      if (C.countr_zero() < Amt)
+        return nullptr;
+      Type *TruncTy = ShType->getWithNewBitWidth(TypeBits - Amt);
+      Constant *NewC =
+          ConstantInt::get(TruncTy, C.ashr(*ShiftAmt).trunc(TypeBits - Amt));
+      return new ICmpInst(
+          Pred, Builder.CreateTrunc(X, TruncTy, "", Shl->hasNoSignedWrap()),
----------------
dtcxzyw wrote:

```
----------------------------------------
define i1 @src_eq_nuw(i32 %x, i32 %c) {
#0:
  %cttz = cttz i32 %c, 1
  %cond = icmp uge i32 %cttz, 16
  assume i1 %cond
  %shl = shl nuw i32 %x, 16
  %icmp = icmp eq i32 %shl, %c
  ret i1 %icmp
}
=>
define i1 @tgt_eq_nuw(i32 %x, i32 %c) {
#0:
  %trunc = trunc i32 %x to i16
  %ashr_c = ashr i32 %c, 16
  %trunc_c = trunc nuw i32 %ashr_c to i16
  %icmp = icmp eq i16 %trunc, %trunc_c
  ret i1 %icmp
}
Transformation doesn't verify!

ERROR: Target is more poisonous than source

Example:
i32 %x = #x00000000 (0)
i32 %c = #xbfff0000 (3221159936, -1073807360)

Source:
i32 %cttz = #x00000010 (16)
i1 %cond = #x1 (1)
i32 %shl = #x00000000 (0)
i1 %icmp = #x0 (0)

Target:
i16 %trunc = #x0000 (0)
i32 %ashr_c = #xffffbfff (4294950911, -16385)
i16 %trunc_c = poison
i1 %icmp = poison
Source value: #x0 (0)
Target value: poison


----------------------------------------
define i1 @src_ult_nuw(i32 %x, i32 %c) {
#0:
  %cttz = cttz i32 %c, 1
  %cond = icmp uge i32 %cttz, 16
  assume i1 %cond
  %shl = shl nuw i32 %x, 16
  %icmp = icmp ult i32 %shl, %c
  ret i1 %icmp
}
=>
define i1 @tgt_ult_nuw(i32 %x, i32 %c) {
#0:
  %trunc = trunc i32 %x to i16
  %ashr_c = ashr i32 %c, 16
  %trunc_c = trunc nuw i32 %ashr_c to i16
  %icmp = icmp ult i16 %trunc, %trunc_c
  ret i1 %icmp
}
Transformation doesn't verify!

ERROR: Target is more poisonous than source

Example:
i32 %x = #x00000000 (0)
i32 %c = #xbfff0000 (3221159936, -1073807360)

Source:
i32 %cttz = #x00000010 (16)
i1 %cond = #x1 (1)
i32 %shl = #x00000000 (0)
i1 %icmp = #x1 (1)

Target:
i16 %trunc = #x0000 (0)
i32 %ashr_c = #xffffbfff (4294950911, -16385)
i16 %trunc_c = poison
i1 %icmp = poison
Source value: #x1 (1)
Target value: poison


----------------------------------------
define i1 @src_slt_nuw(i32 %x, i32 %c) {
#0:
  %cttz = cttz i32 %c, 1
  %cond = icmp uge i32 %cttz, 16
  assume i1 %cond
  %shl = shl nuw i32 %x, 16
  %icmp = icmp slt i32 %shl, %c
  ret i1 %icmp
}
=>
define i1 @tgt_slt_nuw(i32 %x, i32 %c) {
#0:
  %trunc = trunc i32 %x to i16
  %ashr_c = ashr i32 %c, 16
  %trunc_c = trunc nuw i32 %ashr_c to i16
  %icmp = icmp slt i16 %trunc, %trunc_c
  ret i1 %icmp
}
Transformation doesn't verify!

ERROR: Target is more poisonous than source

Example:
i32 %x = #x00000000 (0)
i32 %c = #xbfff0000 (3221159936, -1073807360)

Source:
i32 %cttz = #x00000010 (16)
i1 %cond = #x1 (1)
i32 %shl = #x00000000 (0)
i1 %icmp = #x0 (0)

Target:
i16 %trunc = #x0000 (0)
i32 %ashr_c = #xffffbfff (4294950911, -16385)
i16 %trunc_c = poison
i1 %icmp = poison
Source value: #x0 (0)
Target value: poison
```



https://github.com/llvm/llvm-project/pull/92773