[PATCH] D100170: [NARY] Don't optimize min/max if there are side uses

[Evgeniy via Phabricator via llvm-commits]

ebrevnov created this revision.
Herald added subscribers: hiraditya, kristof.beyls.
ebrevnov requested review of this revision.
Herald added a project: LLVM.
Herald added a subscriber: llvm-commits.

Say we have
%1=min(%a,%b)
%2=min(%b,%c)
%3=min(%2,%a)

The optimization will try to reassociate the later one so that we can rewrite it to %3=min(%1, %c) and remove %2.
But if %2 has another uses outside of %3 then we can't remove %2 and end up with:

%1=min(%a,%b)
%2=min(%b,%c)
%3=min(%1, %c)

This doesn't harm by itself except it is not profitable and changes IR for no good reason.
What is bad it triggers next iteration which finds out that optimization is applicable to %2 and %3 and generates:

%1=min(%a,%b)
%2=min(%b,%c)
%3=min(%1,%c)
%4=min(%2,%a)

and so on...

The solution is to prevent optimization in the first place if intermediate result (%2) has side uses and
known to be not removed.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D100170

Files:
  llvm/lib/Transforms/Scalar/NaryReassociate.cpp
  llvm/test/Transforms/NaryReassociate/nary-req.ll


Index: llvm/test/Transforms/NaryReassociate/nary-req.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/NaryReassociate/nary-req.ll
@@ -0,0 +1,32 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -nary-reassociate -S | FileCheck %s
+; RUN: opt < %s -passes='nary-reassociate' -S | FileCheck %s
+
+declare i32 @llvm.smax.i32(i32 %a, i32 %b)
+
+; This is a negative test. We should not optimize if intermediate result
+; has a use outside of optimizaple pattern. In other words %smax2 has one
+; use from %smax3 and side use from %res2.
+define i32 @smax_test1(i32 %a, i32 %b, i32 %c) {
+; CHECK-LABEL: @smax_test1(
+; CHECK-NEXT:    [[C1:%.*]] = icmp sgt i32 [[A:%.*]], [[B:%.*]]
+; CHECK-NEXT:    [[SMAX1:%.*]] = select i1 [[C1]], i32 [[A]], i32 [[B]]
+; CHECK-NEXT:    [[C2:%.*]] = icmp sgt i32 [[B]], [[C:%.*]]
+; CHECK-NEXT:    [[SMAX2:%.*]] = select i1 [[C2]], i32 [[B]], i32 [[C]]
+; CHECK-NEXT:    [[C3:%.*]] = icmp sgt i32 [[SMAX2]], [[A]]
+; CHECK-NEXT:    [[SMAX3:%.*]] = select i1 [[C3]], i32 [[SMAX2]], i32 [[A]]
+; CHECK-NEXT:    [[RES:%.*]] = add i32 [[SMAX1]], [[SMAX3]]
+; CHECK-NEXT:    [[RES2:%.*]] = add i32 [[RES]], [[SMAX2]]
+; CHECK-NEXT:    ret i32 [[RES]]
+;
+  %c1 = icmp sgt i32 %a, %b
+  %smax1 = select i1 %c1, i32 %a, i32 %b
+  %c2 = icmp sgt i32 %b, %c
+  %smax2 = select i1 %c2, i32 %b, i32 %c
+  %c3 = icmp sgt i32 %smax2, %a
+  %smax3 = select i1 %c3, i32 %smax2, i32 %a
+  %res = add i32 %smax1, %smax3
+  %res2 = add i32 %res, %smax2
+  ret i32 %res
+}
+
Index: llvm/lib/Transforms/Scalar/NaryReassociate.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/NaryReassociate.cpp
+++ llvm/lib/Transforms/Scalar/NaryReassociate.cpp
@@ -592,7 +592,7 @@
   Value *A = nullptr, *B = nullptr;
   MaxMinT m_MaxMin(m_Value(A), m_Value(B));
   for (unsigned int i = 0; i < 2; ++i) {
-    if (match(LHS, m_MaxMin)) {
+    if (LHS->getNumUses() <= 2 && match(LHS, m_MaxMin)) {
       const SCEV *AExpr = SE->getSCEV(A), *BExpr = SE->getSCEV(B);
       const SCEV *RHSExpr = SE->getSCEV(RHS);
       for (unsigned int j = 0; j < 2; ++j) {
@@ -610,6 +610,12 @@
           std::swap(AExpr, RHSExpr);
         }
 
+        // The optimization is profitable only if LHS can be removed in the end.
+        // In other words LHS should be used (directly or indirectly) by I only.
+        for (User *U : LHS->users())
+          if (U != I || !(U->hasOneUser() && *U->users().begin() == I))
+            continue;
+
         SCEVExpander Expander(*SE, *DL, "nary-reassociate");
         SmallVector<const SCEV *, 2> Ops1{ BExpr, AExpr };
         const SCEVTypes SCEVType = convertToSCEVype(m_MaxMin);


-------------- next part --------------
A non-text attachment was scrubbed...
Name: D100170.336375.patch
Type: text/x-patch
Size: 2772 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20210409/75b7f6e9/attachment.bin>