[SCCP] Strengthen two-instruction range checks

[dtcxzyw]

This patch implements the todo discussed in #158495 (comment).
It also fixes a regression introduced by #161000. See also dtcxzyw/llvm-opt-benchmark#2890 (comment).

IR diff: dtcxzyw/llvm-opt-benchmark#2892

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-function-specialization

Author: Yingwei Zheng (dtcxzyw)

Changes

This patch implements the todo discussed in #158495 (comment).
It also fixes a regression introduced by #161000. See also dtcxzyw/llvm-opt-benchmark#2890 (comment).

IR diff: dtcxzyw/llvm-opt-benchmark#2892

---

Full diff: https://github.com/llvm/llvm-project/pull/162008.diff

2 Files Affected:

• (modified) llvm/lib/Transforms/Utils/SCCPSolver.cpp (+22-17)
• (modified) llvm/test/Transforms/SCCP/relax-range-checks.ll (+24)

diff --git a/llvm/lib/Transforms/Utils/SCCPSolver.cpp b/llvm/lib/Transforms/Utils/SCCPSolver.cpp
index af216cd9214bf..2e8e5ba18d933 100644
--- a/llvm/lib/Transforms/Utils/SCCPSolver.cpp
+++ b/llvm/lib/Transforms/Utils/SCCPSolver.cpp
@@ -317,24 +317,29 @@ static Value *simplifyInstruction(SCCPSolver &Solver,
       // Early exit if we know nothing about X.
       if (LRange.isFullSet())
         return nullptr;
-      // We are allowed to refine the comparison to either true or false for out
-      // of range inputs. Here we refine the comparison to true, i.e. we relax
-      // the range check.
-      auto NewCR = CR->exactUnionWith(LRange.inverse());
-      // TODO: Check if we can narrow the range check to an equality test.
-      // E.g, for X in [0, 4), X - 3 u< 2 -> X == 3
-      if (!NewCR)
+      auto ConvertCRToICmp =
+          [&](const std::optional<ConstantRange> &NewCR) -> Value * {
+        ICmpInst::Predicate Pred;
+        APInt RHS;
+        // Check if we can represent NewCR as an icmp predicate.
+        if (NewCR && NewCR->getEquivalentICmp(Pred, RHS)) {
+          IRBuilder<NoFolder> Builder(&Inst);
+          Value *NewICmp =
+              Builder.CreateICmp(Pred, X, ConstantInt::get(X->getType(), RHS));
+          InsertedValues.insert(NewICmp);
+          return NewICmp;
+        }
         return nullptr;
-      ICmpInst::Predicate Pred;
-      APInt RHS;
-      // Check if we can represent NewCR as an icmp predicate.
-      if (NewCR->getEquivalentICmp(Pred, RHS)) {
-        IRBuilder<NoFolder> Builder(&Inst);
-        Value *NewICmp =
-            Builder.CreateICmp(Pred, X, ConstantInt::get(X->getType(), RHS));
-        InsertedValues.insert(NewICmp);
-        return NewICmp;
-      }
+      };
+      // We are allowed to refine the comparison to either true or false for out
+      // of range inputs.
+      // Here we refine the comparison to false, and check if we can narrow the
+      // range check to an equality test.
+      if (auto *V = ConvertCRToICmp(CR->exactIntersectWith(LRange)))
+        return V;
+      // Here we refine the comparison to true, i.e. we relax the range check.
+      if (auto *V = ConvertCRToICmp(CR->exactUnionWith(LRange.inverse())))
+        return V;
     }
   }
 
diff --git a/llvm/test/Transforms/SCCP/relax-range-checks.ll b/llvm/test/Transforms/SCCP/relax-range-checks.ll
index 90722f350aa9e..34e48136df37a 100644
--- a/llvm/test/Transforms/SCCP/relax-range-checks.ll
+++ b/llvm/test/Transforms/SCCP/relax-range-checks.ll
@@ -89,4 +89,28 @@ define i1 @relax_range_check_multiuse(i8 range(i8 0, 5) %x)  {
   ret i1 %ret
 }
 
+define i1 @range_check_to_icmp_eq1(i32 range(i32 0, 4) %x) {
+; CHECK-LABEL: define i1 @range_check_to_icmp_eq1(
+; CHECK-SAME: i32 range(i32 0, 4) [[X:%.*]]) {
+; CHECK-NEXT:    [[OFF:%.*]] = add nsw i32 [[X]], -3
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp eq i32 [[X]], 3
+; CHECK-NEXT:    ret i1 [[TMP1]]
+;
+  %off = add nsw i32 %x, -3
+  %cmp = icmp ult i32 %off, 2
+  ret i1 %cmp
+}
+
+define i1 @range_check_to_icmp_eq2(i32 range(i32 -1, 2) %x) {
+; CHECK-LABEL: define i1 @range_check_to_icmp_eq2(
+; CHECK-SAME: i32 range(i32 -1, 2) [[X:%.*]]) {
+; CHECK-NEXT:    [[OFF:%.*]] = add nsw i32 [[X]], -1
+; CHECK-NEXT:    [[CMP:%.*]] = icmp eq i32 [[X]], 1
+; CHECK-NEXT:    ret i1 [[CMP]]
+;
+  %off = add nsw i32 %x, -1
+  %cmp = icmp ult i32 %off, -2
+  ret i1 %cmp
+}
+
 declare void @use(i8)

[dianqk]

The diff sounds reasonable to me.

[andjo403]

Looks good to me

[nikic]

LGTM. This can be further extended to use a mix of true and false for the out of range part.