[llvm] ConstraintElim: add dry-run routine to fail early (PR #99670)

[Florian Hahn via llvm-commits]

fhahn wrote:

I had a chance to check SPEC2017 and there are a few cases where fewer conditions are simplified (with -O3)

```
Program                                       constraint-elimination.NumCondsRemoved
                                              base                                   patch   diff
External/S...NT2017rate/502.gcc_r/502.gcc_r   1123.00                                1098.00  -2.2%
External/S...rate/510.parest_r/510.parest_r    273.00                                 262.00  -4.0%
External/S...00.perlbench_r/500.perlbench_r     97.00                                  88.00  -9.3%
External/S...te/538.imagick_r/538.imagick_r     30.00                                  24.00 -20.0%
External/S...te/526.blender_r/526.blender_r    599.00                                 344.00 -42.6%
```

In Sqlite from the test suite, there also are a few cases where we have fewer simplifications (but no binary changes probably due to them being simplified later).

I am wondering if we should aim for a check like below, to surface more cases where the behavior changes (with that change, I also tried bootstrapping Clang but couldn't get very far)

```
diff --git a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
index 40d117d49489..916f90c1fa0f 100644
--- a/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
+++ b/llvm/lib/Transforms/Scalar/ConstraintElimination.cpp
@@ -1898,8 +1898,8 @@ static bool eliminateConstraints(Function &F, DominatorTree &DT, LoopInfo &LI,

   // Fail early if estimates exceed limits. Row estimate could be off by up to
   // 40%.
-  if (EstimatedRows > 1.4 * MaxRows || EstimatedColumns > MaxColumns)
-    return false;
+  // if (EstimatedRows > 1.4 * MaxRows || EstimatedColumns > MaxColumns)
+  //  return false;

   SmallVector<Value *> FunctionArgs;
   for (Value &Arg : F.args())
@@ -2058,6 +2058,9 @@ static bool eliminateConstraints(Function &F, DominatorTree &DT, LoopInfo &LI,

   for (Instruction *I : ToRemove)
     I->eraseFromParent();
+
+  assert(!Changed ||
+         (EstimatedRows <= 1.4 * MaxRows && EstimatedColumns <= MaxColumns));
   return Changed;
 }

```

https://github.com/llvm/llvm-project/pull/99670