[llvm] [SCEV] Use context sensative reasoning in howFarToZero (PR #94525)
Philip Reames via llvm-commits
llvm-commits at lists.llvm.org
Wed Jun 5 12:24:16 PDT 2024
https://github.com/preames created https://github.com/llvm/llvm-project/pull/94525
This change builds on 0a357ad which supported non-constant strides in howFarToZero, but used only context insensative reasoning.
This change does two things:
1) Directly use context sensative queries to prove facts established
before the loop. Note that we technically only need facts known
at the latch, but using facts known on entry is a conservative
approximation which will cover most everything.
2) For the non-zero check, we can usually prove non-zero from the
finite assumption implied by mustprogress. This eliminates the
need to do the context sensative query in the common case.
>From 215810d3d72e0f211f3f2537f0827e238ad3ff9c Mon Sep 17 00:00:00 2001
From: Philip Reames <preames at rivosinc.com>
Date: Wed, 5 Jun 2024 12:19:04 -0700
Subject: [PATCH] [SCEV] Use context sensative reasoning in howFarToZero
This change builds on 0a357ad which supported non-constant strides in
howFarToZero, but used only context insensative reasoning.
This change does two things:
1) Directly use context sensative queries to prove facts established
before the loop. Note that we technically only need facts known
at the latch, but using facts known on entry is a conservative
approximation which will cover most everything.
2) For the non-zero check, we can usually prove non-zero from the
finite assumption implied by mustprogress. This eliminates the
need to do the context sensative query in the common case.
---
llvm/lib/Analysis/ScalarEvolution.cpp | 19 ++++++---
.../max-backedge-taken-count-guard-info.ll | 8 ++--
.../trip-count-unknown-stride.ll | 42 +++++++++++--------
llvm/test/Transforms/LoopRotate/pr56260.ll | 11 +----
4 files changed, 43 insertions(+), 37 deletions(-)
diff --git a/llvm/lib/Analysis/ScalarEvolution.cpp b/llvm/lib/Analysis/ScalarEvolution.cpp
index 9808308cbfed9..8b947870e6da4 100644
--- a/llvm/lib/Analysis/ScalarEvolution.cpp
+++ b/llvm/lib/Analysis/ScalarEvolution.cpp
@@ -10485,7 +10485,7 @@ ScalarEvolution::ExitLimit ScalarEvolution::howFarToZero(const SCEV *V,
const SCEV *Step = getSCEVAtScope(AddRec->getOperand(1), L->getParentLoop());
const SCEVConstant *StepC = dyn_cast<SCEVConstant>(Step);
- if (!isLoopInvariant(Step, L) || !isKnownNonZero(Step))
+ if (!isLoopInvariant(Step, L))
return getCouldNotCompute();
// For positive steps (counting up until unsigned overflow):
@@ -10493,8 +10493,10 @@ ScalarEvolution::ExitLimit ScalarEvolution::howFarToZero(const SCEV *V,
// For negative steps (counting down to zero):
// N = Start/-Step
// First compute the unsigned distance from zero in the direction of Step.
- bool CountDown = isKnownNegative(Step);
- if (!CountDown && !isKnownNonNegative(Step))
+ const SCEV *Zero = getZero(AddRec->getType());
+ bool CountDown = isLoopEntryGuardedByCond(L, ICmpInst::ICMP_SLT, Step, Zero);
+ if (!CountDown &&
+ !isLoopEntryGuardedByCond(L, ICmpInst::ICMP_SGE, Step, Zero))
return getCouldNotCompute();
const SCEV *Distance = CountDown ? Start : getNegativeSCEV(Start);
@@ -10513,7 +10515,6 @@ ScalarEvolution::ExitLimit ScalarEvolution::howFarToZero(const SCEV *V,
// Explicitly handling this here is necessary because getUnsignedRange
// isn't context-sensitive; it doesn't know that we only care about the
// range inside the loop.
- const SCEV *Zero = getZero(Distance->getType());
const SCEV *One = getOne(Distance->getType());
const SCEV *DistancePlusOne = getAddExpr(Distance, One);
if (isLoopEntryGuardedByCond(L, ICmpInst::ICMP_NE, DistancePlusOne, Zero)) {
@@ -10533,6 +10534,14 @@ ScalarEvolution::ExitLimit ScalarEvolution::howFarToZero(const SCEV *V,
// will have undefined behavior due to wrapping.
if (ControlsOnlyExit && AddRec->hasNoSelfWrap() &&
loopHasNoAbnormalExits(AddRec->getLoop())) {
+
+ // If the stride is zero, the loop must be infinite. Most loops are
+ // finite by assumption, in which case the step being zero implies UB
+ // must execute if the loop is entered.
+ if (!loopIsFiniteByAssumption(L) &&
+ !isLoopEntryGuardedByCond(L, ICmpInst::ICMP_NE, Step, Zero))
+ return getCouldNotCompute();
+
const SCEV *Exact =
getUDivExpr(Distance, CountDown ? getNegativeSCEV(Step) : Step);
const SCEV *ConstantMax = getCouldNotCompute();
@@ -10547,7 +10556,7 @@ ScalarEvolution::ExitLimit ScalarEvolution::howFarToZero(const SCEV *V,
}
// Solve the general equation.
- if (!StepC)
+ if (!StepC || StepC->getValue()->isZero())
return getCouldNotCompute();
const SCEV *E = SolveLinEquationWithOverflow(StepC->getAPInt(),
getNegativeSCEV(Start), *this);
diff --git a/llvm/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info.ll b/llvm/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info.ll
index 413bd21554c98..15e672d186c26 100644
--- a/llvm/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info.ll
+++ b/llvm/test/Analysis/ScalarEvolution/max-backedge-taken-count-guard-info.ll
@@ -918,13 +918,13 @@ define void @crash(ptr %ptr) {
; CHECK-NEXT: %incdec.ptr112 = getelementptr inbounds i8, ptr %text.addr.5, i64 -1
; CHECK-NEXT: --> {(-1 + null)<nuw><nsw>,+,-1}<nw><%while.cond111> U: full-set S: full-set Exits: <<Unknown>> LoopDispositions: { %while.cond111: Computable, %while.body: Variant }
; CHECK-NEXT: %lastout.2271 = phi ptr [ %incdec.ptr126, %while.body125 ], [ %ptr, %while.end117 ]
-; CHECK-NEXT: --> {%ptr,+,1}<nuw><%while.body125> U: full-set S: full-set Exits: {(-2 + (-1 * (ptrtoint ptr %ptr to i64)) + %ptr),+,-1}<nw><%while.cond111> LoopDispositions: { %while.body125: Computable }
+; CHECK-NEXT: --> {%ptr,+,1}<nuw><%while.body125> U: full-set S: full-set Exits: {(2 + (ptrtoint ptr %ptr to i64) + %ptr),+,1}<nw><%while.cond111> LoopDispositions: { %while.body125: Computable }
; CHECK-NEXT: %incdec.ptr126 = getelementptr inbounds i8, ptr %lastout.2271, i64 1
-; CHECK-NEXT: --> {(1 + %ptr),+,1}<nuw><%while.body125> U: full-set S: full-set Exits: {(-1 + (-1 * (ptrtoint ptr %ptr to i64)) + %ptr),+,-1}<nw><%while.cond111> LoopDispositions: { %while.body125: Computable }
+; CHECK-NEXT: --> {(1 + %ptr),+,1}<nuw><%while.body125> U: full-set S: full-set Exits: {(3 + (ptrtoint ptr %ptr to i64) + %ptr),+,1}<nw><%while.cond111> LoopDispositions: { %while.body125: Computable }
; CHECK-NEXT: Determining loop execution counts for: @crash
-; CHECK-NEXT: Loop %while.body125: backedge-taken count is {(-2 + (-1 * (ptrtoint ptr %ptr to i64))),+,-1}<nw><%while.cond111>
+; CHECK-NEXT: Loop %while.body125: backedge-taken count is {(2 + (ptrtoint ptr %ptr to i64)),+,1}<nw><%while.cond111>
; CHECK-NEXT: Loop %while.body125: constant max backedge-taken count is i64 -2
-; CHECK-NEXT: Loop %while.body125: symbolic max backedge-taken count is {(-2 + (-1 * (ptrtoint ptr %ptr to i64))),+,-1}<nw><%while.cond111>
+; CHECK-NEXT: Loop %while.body125: symbolic max backedge-taken count is {(2 + (ptrtoint ptr %ptr to i64)),+,1}<nw><%while.cond111>
; CHECK-NEXT: Loop %while.body125: Trip multiple is 1
; CHECK-NEXT: Loop %while.cond111: Unpredictable backedge-taken count.
; CHECK-NEXT: Loop %while.cond111: Unpredictable constant max backedge-taken count.
diff --git a/llvm/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll b/llvm/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll
index ecf13320a5e81..2d02cb6194f4c 100644
--- a/llvm/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll
+++ b/llvm/test/Analysis/ScalarEvolution/trip-count-unknown-stride.ll
@@ -271,9 +271,10 @@ define void @ne_nsw_pos_step(ptr nocapture %A, i32 %n, i32 %s) mustprogress {
;
; CHECK-LABEL: 'ne_nsw_pos_step'
; CHECK-NEXT: Determining loop execution counts for: @ne_nsw_pos_step
-; CHECK-NEXT: Loop %for.body: Unpredictable backedge-taken count.
-; CHECK-NEXT: Loop %for.body: Unpredictable constant max backedge-taken count.
-; CHECK-NEXT: Loop %for.body: Unpredictable symbolic max backedge-taken count.
+; CHECK-NEXT: Loop %for.body: backedge-taken count is (((-1 * %s) + %n) /u %s)
+; CHECK-NEXT: Loop %for.body: constant max backedge-taken count is i32 -1
+; CHECK-NEXT: Loop %for.body: symbolic max backedge-taken count is (((-1 * %s) + %n) /u %s)
+; CHECK-NEXT: Loop %for.body: Trip multiple is 1
;
entry:
%pos_step = icmp sgt i32 %s, 0
@@ -299,9 +300,10 @@ define void @ne_nsw_neg_step(ptr nocapture %A, i32 %n, i32 %s) mustprogress {
;
; CHECK-LABEL: 'ne_nsw_neg_step'
; CHECK-NEXT: Determining loop execution counts for: @ne_nsw_neg_step
-; CHECK-NEXT: Loop %for.body: Unpredictable backedge-taken count.
-; CHECK-NEXT: Loop %for.body: Unpredictable constant max backedge-taken count.
-; CHECK-NEXT: Loop %for.body: Unpredictable symbolic max backedge-taken count.
+; CHECK-NEXT: Loop %for.body: backedge-taken count is (((-1 * %n) + %s) /u (-1 * %s))
+; CHECK-NEXT: Loop %for.body: constant max backedge-taken count is i32 -2
+; CHECK-NEXT: Loop %for.body: symbolic max backedge-taken count is (((-1 * %n) + %s) /u (-1 * %s))
+; CHECK-NEXT: Loop %for.body: Trip multiple is 1
;
entry:
%neg_step = icmp slt i32 %s, 0
@@ -327,9 +329,10 @@ define void @ne_nsw_nonneg_step(ptr nocapture %A, i32 %n, i32 %s) mustprogress {
;
; CHECK-LABEL: 'ne_nsw_nonneg_step'
; CHECK-NEXT: Determining loop execution counts for: @ne_nsw_nonneg_step
-; CHECK-NEXT: Loop %for.body: Unpredictable backedge-taken count.
-; CHECK-NEXT: Loop %for.body: Unpredictable constant max backedge-taken count.
-; CHECK-NEXT: Loop %for.body: Unpredictable symbolic max backedge-taken count.
+; CHECK-NEXT: Loop %for.body: backedge-taken count is (((-1 * %s) + %n) /u %s)
+; CHECK-NEXT: Loop %for.body: constant max backedge-taken count is i32 -1
+; CHECK-NEXT: Loop %for.body: symbolic max backedge-taken count is (((-1 * %s) + %n) /u %s)
+; CHECK-NEXT: Loop %for.body: Trip multiple is 1
;
entry:
%nonneg_step = icmp sge i32 %s, 0
@@ -381,9 +384,10 @@ define void @ne_nuw_pos_step(ptr nocapture %A, i32 %n, i32 %s) mustprogress {
;
; CHECK-LABEL: 'ne_nuw_pos_step'
; CHECK-NEXT: Determining loop execution counts for: @ne_nuw_pos_step
-; CHECK-NEXT: Loop %for.body: Unpredictable backedge-taken count.
-; CHECK-NEXT: Loop %for.body: Unpredictable constant max backedge-taken count.
-; CHECK-NEXT: Loop %for.body: Unpredictable symbolic max backedge-taken count.
+; CHECK-NEXT: Loop %for.body: backedge-taken count is (((-1 * %s) + %n) /u %s)
+; CHECK-NEXT: Loop %for.body: constant max backedge-taken count is i32 -1
+; CHECK-NEXT: Loop %for.body: symbolic max backedge-taken count is (((-1 * %s) + %n) /u %s)
+; CHECK-NEXT: Loop %for.body: Trip multiple is 1
;
entry:
%pos_step = icmp sgt i32 %s, 0
@@ -409,9 +413,10 @@ define void @ne_nuw_neg_step(ptr nocapture %A, i32 %n, i32 %s) mustprogress {
;
; CHECK-LABEL: 'ne_nuw_neg_step'
; CHECK-NEXT: Determining loop execution counts for: @ne_nuw_neg_step
-; CHECK-NEXT: Loop %for.body: Unpredictable backedge-taken count.
-; CHECK-NEXT: Loop %for.body: Unpredictable constant max backedge-taken count.
-; CHECK-NEXT: Loop %for.body: Unpredictable symbolic max backedge-taken count.
+; CHECK-NEXT: Loop %for.body: backedge-taken count is (((-1 * %n) + %s) /u (-1 * %s))
+; CHECK-NEXT: Loop %for.body: constant max backedge-taken count is i32 -2
+; CHECK-NEXT: Loop %for.body: symbolic max backedge-taken count is (((-1 * %n) + %s) /u (-1 * %s))
+; CHECK-NEXT: Loop %for.body: Trip multiple is 1
;
entry:
%neg_step = icmp slt i32 %s, 0
@@ -437,9 +442,10 @@ define void @ne_nuw_nonneg_step(ptr nocapture %A, i32 %n, i32 %s) mustprogress {
;
; CHECK-LABEL: 'ne_nuw_nonneg_step'
; CHECK-NEXT: Determining loop execution counts for: @ne_nuw_nonneg_step
-; CHECK-NEXT: Loop %for.body: Unpredictable backedge-taken count.
-; CHECK-NEXT: Loop %for.body: Unpredictable constant max backedge-taken count.
-; CHECK-NEXT: Loop %for.body: Unpredictable symbolic max backedge-taken count.
+; CHECK-NEXT: Loop %for.body: backedge-taken count is (((-1 * %s) + %n) /u %s)
+; CHECK-NEXT: Loop %for.body: constant max backedge-taken count is i32 -1
+; CHECK-NEXT: Loop %for.body: symbolic max backedge-taken count is (((-1 * %s) + %n) /u %s)
+; CHECK-NEXT: Loop %for.body: Trip multiple is 1
;
entry:
%nonneg_step = icmp sge i32 %s, 0
diff --git a/llvm/test/Transforms/LoopRotate/pr56260.ll b/llvm/test/Transforms/LoopRotate/pr56260.ll
index 70b68e7cf6db3..f9772eed4560f 100644
--- a/llvm/test/Transforms/LoopRotate/pr56260.ll
+++ b/llvm/test/Transforms/LoopRotate/pr56260.ll
@@ -17,16 +17,7 @@ define void @main() {
; CHECK-NEXT: [[TOBOOL3_NOT1:%.*]] = icmp eq i32 [[INC]], 0
; CHECK-NEXT: br i1 [[TOBOOL3_NOT1]], label [[L0_PREHEADER_LOOPEXIT]], label [[L1_PREHEADER_LR_PH:%.*]]
; CHECK: L1.preheader.lr.ph:
-; CHECK-NEXT: br label [[L1_PREHEADER:%.*]]
-; CHECK: L1.preheader:
-; CHECK-NEXT: [[SPEC_SELECT3:%.*]] = phi i32 [ [[INC]], [[L1_PREHEADER_LR_PH]] ], [ [[SPEC_SELECT:%.*]], [[L0_LATCH:%.*]] ]
-; CHECK-NEXT: [[K_02:%.*]] = phi i32 [ 0, [[L1_PREHEADER_LR_PH]] ], [ [[SPEC_SELECT3]], [[L0_LATCH]] ]
-; CHECK-NEXT: [[TOBOOL8_NOT:%.*]] = icmp eq i32 [[K_02]], 0
-; CHECK-NEXT: br label [[L0_LATCH]]
-; CHECK: L0.latch:
-; CHECK-NEXT: [[SPEC_SELECT]] = add nsw i32 [[SPEC_SELECT3]], [[INC]]
-; CHECK-NEXT: [[TOBOOL3_NOT:%.*]] = icmp eq i32 [[SPEC_SELECT]], 0
-; CHECK-NEXT: br i1 [[TOBOOL3_NOT]], label [[L0_L0_PREHEADER_LOOPEXIT_CRIT_EDGE:%.*]], label [[L1_PREHEADER]]
+; CHECK-NEXT: br label [[L0_L0_PREHEADER_LOOPEXIT_CRIT_EDGE:%.*]]
;
entry:
br label %L0.preheader
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