[llvm] [SCEV] Preserve flags in SCEVLoopGuardRewriter for add and mul. (PR #91472)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: Florian Hahn (fhahn)

<details>
<summary>Changes</summary>

SCEVLoopGuardRewriter only replaces operands with equivalent values, so we should be able to transfer the flags from the original expression.

---
Full diff: https://github.com/llvm/llvm-project/pull/91472.diff


3 Files Affected:

- (modified) llvm/lib/Analysis/ScalarEvolution.cpp (+24) 
- (modified) llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info.ll (+4-4) 
- (modified) llvm/test/Transforms/IndVarSimplify/trip-count-expansion-loop-guard-preserve-nsw.ll (+1-2) 


``````````diff
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 93f885c5d5ad8..3666a7d4b9b22 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -15034,6 +15034,30 @@ class SCEVLoopGuardRewriter : public SCEVRewriteVisitor<SCEVLoopGuardRewriter> {
       return SCEVRewriteVisitor<SCEVLoopGuardRewriter>::visitSMinExpr(Expr);
     return I->second;
   }
+
+  const SCEV *visitAddExpr(const SCEVAddExpr *Expr) {
+    SmallVector<const SCEV *, 2> Operands;
+    bool Changed = false;
+    for (const auto *Op : Expr->operands()) {
+      Operands.push_back(SCEVRewriteVisitor<SCEVLoopGuardRewriter>::visit(Op));
+      Changed |= Op != Operands.back();
+    }
+    // We are only replacing operands with equivalent values, so transfer the
+    // flags from the original expression.
+    return !Changed ? Expr : SE.getAddExpr(Operands, Expr->getNoWrapFlags());
+  }
+
+  const SCEV *visitMulExpr(const SCEVMulExpr *Expr) {
+    SmallVector<const SCEV *, 2> Operands;
+    bool Changed = false;
+    for (const auto *Op : Expr->operands()) {
+      Operands.push_back(SCEVRewriteVisitor<SCEVLoopGuardRewriter>::visit(Op));
+      Changed |= Op != Operands.back();
+    }
+    // We are only replacing operands with equivalent values, so transfer the
+    // flags from the original expression.
+    return !Changed ? Expr : SE.getMulExpr(Operands, Expr->getNoWrapFlags());
+  }
 };
 
 const SCEV *ScalarEvolution::applyLoopGuards(const SCEV *Expr, const Loop *L) {
diff --git a/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info.ll b/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info.ll
index da4487ce9cd48..3c3748a6a5f02 100644
--- a/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info.ll
+++ b/llvm/test/Analysis/ScalarEvolution/backedge-taken-count-guard-info.ll
@@ -77,13 +77,13 @@ define void @rewrite_preserve_add_nsw(i32 %a) {
 ; CHECK-NEXT:    %add = add nsw i32 %a, 4
 ; CHECK-NEXT:    --> (4 + %a)<nsw> U: [-2147483644,-2147483648) S: [-2147483644,-2147483648)
 ; CHECK-NEXT:    %iv = phi i32 [ 0, %entry ], [ %iv.next, %loop ]
-; CHECK-NEXT:    --> {0,+,1}<nuw><%loop> U: [0,-2147483648) S: [0,-2147483648) Exits: (0 smax (4 + %a)<nsw>) LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {0,+,1}<nuw><%loop> U: [0,-2147483648) S: [0,-2147483648) Exits: (4 + %a)<nsw> LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:    %iv.next = add i32 %iv, 1
-; CHECK-NEXT:    --> {1,+,1}<nuw><%loop> U: [1,-2147483647) S: [1,-2147483647) Exits: (1 + (0 smax (4 + %a)<nsw>))<nuw> LoopDispositions: { %loop: Computable }
+; CHECK-NEXT:    --> {1,+,1}<nuw><%loop> U: [1,-2147483647) S: [1,-2147483647) Exits: (5 + %a) LoopDispositions: { %loop: Computable }
 ; CHECK-NEXT:  Determining loop execution counts for: @rewrite_preserve_add_nsw
-; CHECK-NEXT:  Loop %loop: backedge-taken count is (0 smax (4 + %a)<nsw>)
+; CHECK-NEXT:  Loop %loop: backedge-taken count is (4 + %a)<nsw>
 ; CHECK-NEXT:  Loop %loop: constant max backedge-taken count is i32 2147483647
-; CHECK-NEXT:  Loop %loop: symbolic max backedge-taken count is (0 smax (4 + %a)<nsw>)
+; CHECK-NEXT:  Loop %loop: symbolic max backedge-taken count is (4 + %a)<nsw>
 ; CHECK-NEXT:  Loop %loop: Trip multiple is 1
 ;
 entry:
diff --git a/llvm/test/Transforms/IndVarSimplify/trip-count-expansion-loop-guard-preserve-nsw.ll b/llvm/test/Transforms/IndVarSimplify/trip-count-expansion-loop-guard-preserve-nsw.ll
index f86639ea4c50f..945eb82e18595 100644
--- a/llvm/test/Transforms/IndVarSimplify/trip-count-expansion-loop-guard-preserve-nsw.ll
+++ b/llvm/test/Transforms/IndVarSimplify/trip-count-expansion-loop-guard-preserve-nsw.ll
@@ -12,8 +12,7 @@ define void @rewrite_preserve_add_nsw(i32 %a) {
 ; CHECK-NEXT:    [[PRE:%.*]] = icmp sgt i32 [[A]], -4
 ; CHECK-NEXT:    br i1 [[PRE]], label [[LOOP_PREHEADER:%.*]], label [[EXIT:%.*]]
 ; CHECK:       loop.preheader:
-; CHECK-NEXT:    [[SMAX:%.*]] = call i32 @llvm.smax.i32(i32 [[ADD]], i32 0)
-; CHECK-NEXT:    [[TMP0:%.*]] = add nuw i32 [[SMAX]], 1
+; CHECK-NEXT:    [[TMP0:%.*]] = add i32 [[A]], 5
 ; CHECK-NEXT:    br label [[LOOP:%.*]]
 ; CHECK:       loop:
 ; CHECK-NEXT:    [[IV:%.*]] = phi i32 [ [[IV_NEXT:%.*]], [[LOOP]] ], [ 0, [[LOOP_PREHEADER]] ]

``````````

</details>


https://github.com/llvm/llvm-project/pull/91472