[llvm] [indvars] Always fallback to truncation if AddRec widening fails (PR #70967)
Philip Reames via llvm-commits
llvm-commits at lists.llvm.org
Wed Nov 1 11:13:16 PDT 2023
https://github.com/preames created https://github.com/llvm/llvm-project/pull/70967
The current code structure results in cases where if a) we can't clone the IV user (because it's not in our whitelist) or b) can't prove the SCEV expressions are identical, we'd sometimes leave both the original unwiddened IV and the partially widdened IV in code. Instead, just truncate thw wide IV to the use - same as what we'd do if we couldn't find an addrec to start with.
Noticed this while playing with changing how we produce addrecs. The current structure results in a very tight interlock between SCEVs internal capabilities and indvars code.
>From 4c8dc3609da730a435258f9ea67af690b9beea98 Mon Sep 17 00:00:00 2001
From: Philip Reames <preames at rivosinc.com>
Date: Wed, 1 Nov 2023 10:09:31 -0700
Subject: [PATCH] [indvars] Always fallback to truncation if AddRec widening
fails
The current code structure results in cases where if a) we can't clone the
IV user (because it's not in our whitelist) or b) can't prove the SCEV
expressions are identical, we'd sometimes leave both the original unwiddened
IV and the partially widdened IV in code. Instead, just truncate thw wide
IV to the use - same as what we'd do if we couldn't find an addrec to start
with.
Noticed this while playing with changing how we produce addrecs. The
current structure results in a very tight interlock between SCEVs
internal capabilities and indvars code.
---
llvm/lib/Transforms/Utils/SimplifyIndVar.cpp | 107 +++++++++---------
.../test/Transforms/IndVarSimplify/pr55925.ll | 18 ++-
2 files changed, 64 insertions(+), 61 deletions(-)
diff --git a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
index ae3644183a735bc..52b86aa088e9110 100644
--- a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
@@ -1789,65 +1789,70 @@ Instruction *WidenIV::widenIVUse(WidenIV::NarrowIVDefUse DU, SCEVExpander &Rewri
return nullptr;
}
- // Does this user itself evaluate to a recurrence after widening?
- WidenedRecTy WideAddRec = getExtendedOperandRecurrence(DU);
- if (!WideAddRec.first)
- WideAddRec = getWideRecurrence(DU);
-
- assert((WideAddRec.first == nullptr) ==
- (WideAddRec.second == ExtendKind::Unknown));
- if (!WideAddRec.first) {
- // If use is a loop condition, try to promote the condition instead of
- // truncating the IV first.
- if (widenLoopCompare(DU))
+ auto tryAddRecExpansion = [&]() -> Instruction* {
+ // Does this user itself evaluate to a recurrence after widening?
+ WidenedRecTy WideAddRec = getExtendedOperandRecurrence(DU);
+ if (!WideAddRec.first)
+ WideAddRec = getWideRecurrence(DU);
+ assert((WideAddRec.first == nullptr) ==
+ (WideAddRec.second == ExtendKind::Unknown));
+ if (!WideAddRec.first)
return nullptr;
- // We are here about to generate a truncate instruction that may hurt
- // performance because the scalar evolution expression computed earlier
- // in WideAddRec.first does not indicate a polynomial induction expression.
- // In that case, look at the operands of the use instruction to determine
- // if we can still widen the use instead of truncating its operand.
- if (widenWithVariantUse(DU))
+ // Reuse the IV increment that SCEVExpander created as long as it dominates
+ // NarrowUse.
+ Instruction *WideUse = nullptr;
+ if (WideAddRec.first == WideIncExpr &&
+ Rewriter.hoistIVInc(WideInc, DU.NarrowUse))
+ WideUse = WideInc;
+ else {
+ WideUse = cloneIVUser(DU, WideAddRec.first);
+ if (!WideUse)
+ return nullptr;
+ }
+ // Evaluation of WideAddRec ensured that the narrow expression could be
+ // extended outside the loop without overflow. This suggests that the wide use
+ // evaluates to the same expression as the extended narrow use, but doesn't
+ // absolutely guarantee it. Hence the following failsafe check. In rare cases
+ // where it fails, we simply throw away the newly created wide use.
+ if (WideAddRec.first != SE->getSCEV(WideUse)) {
+ LLVM_DEBUG(dbgs() << "Wide use expression mismatch: " << *WideUse << ": "
+ << *SE->getSCEV(WideUse) << " != " << *WideAddRec.first
+ << "\n");
+ DeadInsts.emplace_back(WideUse);
return nullptr;
+ };
- // This user does not evaluate to a recurrence after widening, so don't
- // follow it. Instead insert a Trunc to kill off the original use,
- // eventually isolating the original narrow IV so it can be removed.
- truncateIVUse(DU, DT, LI);
- return nullptr;
- }
+ // if we reached this point then we are going to replace
+ // DU.NarrowUse with WideUse. Reattach DbgValue then.
+ replaceAllDbgUsesWith(*DU.NarrowUse, *WideUse, *WideUse, *DT);
- // Reuse the IV increment that SCEVExpander created as long as it dominates
- // NarrowUse.
- Instruction *WideUse = nullptr;
- if (WideAddRec.first == WideIncExpr &&
- Rewriter.hoistIVInc(WideInc, DU.NarrowUse))
- WideUse = WideInc;
- else {
- WideUse = cloneIVUser(DU, WideAddRec.first);
- if (!WideUse)
- return nullptr;
- }
- // Evaluation of WideAddRec ensured that the narrow expression could be
- // extended outside the loop without overflow. This suggests that the wide use
- // evaluates to the same expression as the extended narrow use, but doesn't
- // absolutely guarantee it. Hence the following failsafe check. In rare cases
- // where it fails, we simply throw away the newly created wide use.
- if (WideAddRec.first != SE->getSCEV(WideUse)) {
- LLVM_DEBUG(dbgs() << "Wide use expression mismatch: " << *WideUse << ": "
- << *SE->getSCEV(WideUse) << " != " << *WideAddRec.first
- << "\n");
- DeadInsts.emplace_back(WideUse);
+ ExtendKindMap[DU.NarrowUse] = WideAddRec.second;
+ // Returning WideUse pushes it on the worklist.
+ return WideUse;
+ };
+
+ if (auto *I = tryAddRecExpansion())
+ return I;
+
+ // If use is a loop condition, try to promote the condition instead of
+ // truncating the IV first.
+ if (widenLoopCompare(DU))
return nullptr;
- }
- // if we reached this point then we are going to replace
- // DU.NarrowUse with WideUse. Reattach DbgValue then.
- replaceAllDbgUsesWith(*DU.NarrowUse, *WideUse, *WideUse, *DT);
+ // We are here about to generate a truncate instruction that may hurt
+ // performance because the scalar evolution expression computed earlier
+ // in WideAddRec.first does not indicate a polynomial induction expression.
+ // In that case, look at the operands of the use instruction to determine
+ // if we can still widen the use instead of truncating its operand.
+ if (widenWithVariantUse(DU))
+ return nullptr;
- ExtendKindMap[DU.NarrowUse] = WideAddRec.second;
- // Returning WideUse pushes it on the worklist.
- return WideUse;
+ // This user does not evaluate to a recurrence after widening, so don't
+ // follow it. Instead insert a Trunc to kill off the original use,
+ // eventually isolating the original narrow IV so it can be removed.
+ truncateIVUse(DU, DT, LI);
+ return nullptr;
}
/// Add eligible users of NarrowDef to NarrowIVUsers.
diff --git a/llvm/test/Transforms/IndVarSimplify/pr55925.ll b/llvm/test/Transforms/IndVarSimplify/pr55925.ll
index 376e9440acbf22e..420fc209949d4f4 100644
--- a/llvm/test/Transforms/IndVarSimplify/pr55925.ll
+++ b/llvm/test/Transforms/IndVarSimplify/pr55925.ll
@@ -14,14 +14,13 @@ define void @test(ptr %p) personality ptr undef {
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[INDVARS_IV:%.*]] = phi i64 [ [[INDVARS_IV_NEXT:%.*]], [[LOOP_LATCH:%.*]] ], [ 0, [[ENTRY:%.*]] ]
-; CHECK-NEXT: [[IV:%.*]] = phi i32 [ 0, [[ENTRY]] ], [ [[IV_NEXT:%.*]], [[LOOP_LATCH]] ]
-; CHECK-NEXT: [[RES:%.*]] = invoke i32 @foo(i32 returned [[IV]])
+; CHECK-NEXT: [[TMP0:%.*]] = trunc i64 [[INDVARS_IV]] to i32
+; CHECK-NEXT: [[RES:%.*]] = invoke i32 @foo(i32 returned [[TMP0]])
; CHECK-NEXT: to label [[LOOP_LATCH]] unwind label [[EXIT:%.*]]
; CHECK: loop.latch:
; CHECK-NEXT: [[INDVARS_IV_NEXT]] = add i64 [[INDVARS_IV]], 1
-; CHECK-NEXT: [[IV_NEXT]] = add nuw i32 [[IV]], 1
-; CHECK-NEXT: [[TMP0:%.*]] = trunc i64 [[INDVARS_IV]] to i32
-; CHECK-NEXT: [[TMP1:%.*]] = call i32 @foo(i32 [[TMP0]])
+; CHECK-NEXT: [[TMP1:%.*]] = trunc i64 [[INDVARS_IV]] to i32
+; CHECK-NEXT: [[TMP2:%.*]] = call i32 @foo(i32 [[TMP1]])
; CHECK-NEXT: br label [[LOOP]]
; CHECK: exit:
; CHECK-NEXT: [[LP:%.*]] = landingpad { ptr, i32 }
@@ -55,19 +54,18 @@ define void @test_critedge(i1 %c, ptr %p) personality ptr undef {
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[INDVARS_IV:%.*]] = phi i64 [ [[INDVARS_IV_NEXT:%.*]], [[LOOP_LATCH:%.*]] ], [ 0, [[ENTRY:%.*]] ]
-; CHECK-NEXT: [[IV:%.*]] = phi i32 [ 0, [[ENTRY]] ], [ [[IV_NEXT:%.*]], [[LOOP_LATCH]] ]
; CHECK-NEXT: br i1 [[C:%.*]], label [[LOOP_INVOKE:%.*]], label [[LOOP_OTHER:%.*]]
; CHECK: loop.invoke:
; CHECK-NEXT: [[TMP0:%.*]] = trunc i64 [[INDVARS_IV]] to i32
-; CHECK-NEXT: [[RES:%.*]] = invoke i32 @foo(i32 returned [[IV]])
+; CHECK-NEXT: [[TMP1:%.*]] = trunc i64 [[INDVARS_IV]] to i32
+; CHECK-NEXT: [[RES:%.*]] = invoke i32 @foo(i32 returned [[TMP0]])
; CHECK-NEXT: to label [[LOOP_LATCH]] unwind label [[EXIT:%.*]]
; CHECK: loop.other:
; CHECK-NEXT: br label [[LOOP_LATCH]]
; CHECK: loop.latch:
-; CHECK-NEXT: [[PHI:%.*]] = phi i32 [ [[TMP0]], [[LOOP_INVOKE]] ], [ 0, [[LOOP_OTHER]] ]
+; CHECK-NEXT: [[PHI:%.*]] = phi i32 [ [[TMP1]], [[LOOP_INVOKE]] ], [ 0, [[LOOP_OTHER]] ]
; CHECK-NEXT: [[INDVARS_IV_NEXT]] = add i64 [[INDVARS_IV]], 1
-; CHECK-NEXT: [[IV_NEXT]] = add nuw i32 [[IV]], 1
-; CHECK-NEXT: [[TMP1:%.*]] = call i32 @foo(i32 [[PHI]])
+; CHECK-NEXT: [[TMP2:%.*]] = call i32 @foo(i32 [[PHI]])
; CHECK-NEXT: br label [[LOOP]]
; CHECK: exit:
; CHECK-NEXT: [[LP:%.*]] = landingpad { ptr, i32 }
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