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[SCEV] Fix incorrect trip count computation
The way we elide max expressions when computing trip counts is incorrect
-- it breaks cases like this:
```
static int wrapping_add(int a, int b) {
return (int)((unsigned)a + (unsigned)b);
}
void test() {
volatile int end_buf = 2147483548; // INT_MIN - 100
int end = end_buf;
unsigned counter = 0;
for (int start = wrapping_add(end, 200); start < end; start++)
counter++;
print(counter);
}
```
Note: the `NoWrap` variable that was being tested has little to do with
the values flowing into the max expression; it is a property of the
induction variable.
test/Transforms/LoopUnroll/nsw-tripcount.ll was added to solely test
functionality I'm reverting in this change, so I've deleted the test
fully.
llvm-svn: 273079
This commit is contained in:
Sanjoy Das
parent
8ac77ef18f
commit
8576aee95c
@ -8668,18 +8668,8 @@ ScalarEvolution::howManyLessThans(const SCEV *LHS, const SCEV *RHS,
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: ICmpInst::ICMP_ULT;
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const SCEV *Start = IV->getStart();
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const SCEV *End = RHS;
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if (!isLoopEntryGuardedByCond(L, Cond, getMinusSCEV(Start, Stride), RHS)) {
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const SCEV *Diff = getMinusSCEV(RHS, Start);
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// If we have NoWrap set, then we can assume that the increment won't
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// overflow, in which case if RHS - Start is a constant, we don't need to
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// do a max operation since we can just figure it out statically
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if (NoWrap && isa<SCEVConstant>(Diff)) {
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if (cast<SCEVConstant>(Diff)->getAPInt().isNegative())
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End = Start;
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} else
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End = IsSigned ? getSMaxExpr(RHS, Start)
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: getUMaxExpr(RHS, Start);
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}
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if (!isLoopEntryGuardedByCond(L, Cond, getMinusSCEV(Start, Stride), RHS))
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End = IsSigned ? getSMaxExpr(RHS, Start) : getUMaxExpr(RHS, Start);
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const SCEV *BECount = computeBECount(getMinusSCEV(End, Start), Stride, false);
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@ -8754,18 +8744,8 @@ ScalarEvolution::howManyGreaterThans(const SCEV *LHS, const SCEV *RHS,
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const SCEV *Start = IV->getStart();
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const SCEV *End = RHS;
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if (!isLoopEntryGuardedByCond(L, Cond, getAddExpr(Start, Stride), RHS)) {
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const SCEV *Diff = getMinusSCEV(RHS, Start);
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// If we have NoWrap set, then we can assume that the increment won't
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// overflow, in which case if RHS - Start is a constant, we don't need to
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// do a max operation since we can just figure it out statically
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if (NoWrap && isa<SCEVConstant>(Diff)) {
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if (!cast<SCEVConstant>(Diff)->getAPInt().isNegative())
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End = Start;
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} else
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End = IsSigned ? getSMinExpr(RHS, Start)
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: getUMinExpr(RHS, Start);
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}
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if (!isLoopEntryGuardedByCond(L, Cond, getAddExpr(Start, Stride), RHS))
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End = IsSigned ? getSMinExpr(RHS, Start) : getUMinExpr(RHS, Start);
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const SCEV *BECount = computeBECount(getMinusSCEV(Start, End), Stride, false);
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@ -126,7 +126,7 @@ exit:
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}
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; CHECK-LABEL: PR12375
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; CHECK: --> {(4 + %arg)<nsw>,+,4}<nuw><%bb1>{{ U: [^ ]+ S: [^ ]+}}{{ *}}Exits: (8 + %arg)<nsw>
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; CHECK: --> {(4 + %arg)<nsw>,+,4}<nuw><%bb1>{{ U: [^ ]+ S: [^ ]+}}{{ *}}Exits: (4 + (4 * ((-1 + (-1 * %arg) + ((4 + %arg)<nsw> umax (8 + %arg)<nsw>)) /u 4)) + %arg)
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define i32 @PR12375(i32* readnone %arg) {
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bb:
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%tmp = getelementptr inbounds i32, i32* %arg, i64 2
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@ -163,7 +163,7 @@ bb5: ; preds = %bb2
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declare void @f(i32)
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; CHECK-LABEL: nswnowrap
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; CHECK: --> {(1 + %v)<nsw>,+,1}<nsw><%for.body>{{ U: [^ ]+ S: [^ ]+}}{{ *}}Exits: (2 + %v)
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; CHECK: --> {(1 + %v)<nsw>,+,1}<nsw><%for.body>{{ U: [^ ]+ S: [^ ]+}}{{ *}}Exits: (1 + ((1 + %v)<nsw> smax %v))
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define void @nswnowrap(i32 %v, i32* %buf) {
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entry:
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%add = add nsw i32 %v, 1
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81
test/Analysis/ScalarEvolution/trip-count13.ll
Normal file
81
test/Analysis/ScalarEvolution/trip-count13.ll
Normal file
@ -0,0 +1,81 @@
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; RUN: opt -S -analyze -scalar-evolution < %s | FileCheck %s
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define void @u_0(i8 %rhs) {
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; E.g.: %rhs = 255, %start = 99, backedge taken 156 times
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entry:
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%start = add i8 %rhs, 100
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br label %loop
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loop:
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%iv = phi i8 [ %start, %entry ], [ %iv.inc, %loop ]
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%iv.inc = add nuw i8 %iv, 1 ;; Note: this never unsigned-wraps
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%iv.cmp = icmp ult i8 %iv, %rhs
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br i1 %iv.cmp, label %loop, label %leave
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; CHECK-LABEL: Determining loop execution counts for: @u_0
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; CHECK-NEXT: Loop %loop: backedge-taken count is (-100 + (-1 * %rhs) + ((100 + %rhs) umax %rhs))
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; CHECK-NEXT: Loop %loop: max backedge-taken count is -1
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leave:
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ret void
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}
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define void @u_1(i8 %start) {
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entry:
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; E.g.: %start = 99, %rhs = 255, backedge taken 156 times
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%rhs = add i8 %start, -100
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br label %loop
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loop:
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%iv = phi i8 [ %start, %entry ], [ %iv.inc, %loop ]
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%iv.inc = add nuw i8 %iv, 1 ;; Note: this never unsigned-wraps
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%iv.cmp = icmp ult i8 %iv, %rhs
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br i1 %iv.cmp, label %loop, label %leave
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; CHECK-LABEL: Determining loop execution counts for: @u_1
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; CHECK-NEXT: Loop %loop: backedge-taken count is ((-1 * %start) + ((-100 + %start) umax %start))
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; CHECK-NEXT: Loop %loop: max backedge-taken count is -1
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leave:
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ret void
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}
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define void @s_0(i8 %rhs) {
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entry:
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; E.g.: %rhs = 127, %start = -29, backedge taken 156 times
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%start = add i8 %rhs, 100
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br label %loop
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loop:
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%iv = phi i8 [ %start, %entry ], [ %iv.inc, %loop ]
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%iv.inc = add nsw i8 %iv, 1 ;; Note: this never signed-wraps
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%iv.cmp = icmp slt i8 %iv, %rhs
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br i1 %iv.cmp, label %loop, label %leave
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; CHECK-LABEL: Determining loop execution counts for: @s_0
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; CHECK-NEXT: Loop %loop: backedge-taken count is (-100 + (-1 * %rhs) + ((100 + %rhs) smax %rhs))
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; CHECK-NEXT: Loop %loop: max backedge-taken count is -1
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leave:
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ret void
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}
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define void @s_1(i8 %start) {
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entry:
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; E.g.: start = -29, %rhs = 127, %backedge taken 156 times
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%rhs = add i8 %start, -100
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br label %loop
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loop:
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%iv = phi i8 [ %start, %entry ], [ %iv.inc, %loop ]
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%iv.inc = add nsw i8 %iv, 1
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%iv.cmp = icmp slt i8 %iv, %rhs
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br i1 %iv.cmp, label %loop, label %leave
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; CHECK-LABEL: Determining loop execution counts for: @s_1
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; CHECK-NEXT: Loop %loop: backedge-taken count is ((-1 * %start) + ((-100 + %start) smax %start))
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; CHECK-NEXT: Loop %loop: max backedge-taken count is -1
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leave:
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ret void
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}
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@ -1,32 +0,0 @@
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; RUN: opt -loop-unroll -S %s | FileCheck %s
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; extern void f(int);
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; void test1(int v) {
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; for (int i=v; i<=v+1; ++i)
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; f(i);
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; }
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;
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; We can use the nsw information to see that the tripcount will be 2, so the
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; loop should be unrolled as this is always beneficial
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declare void @f(i32)
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; CHECK-LABEL: @test1
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define void @test1(i32 %v) {
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entry:
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%add = add nsw i32 %v, 1
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br label %for.body
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for.body:
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%i.04 = phi i32 [ %v, %entry ], [ %inc, %for.body ]
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tail call void @f(i32 %i.04)
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%inc = add nsw i32 %i.04, 1
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%cmp = icmp slt i32 %i.04, %add
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br i1 %cmp, label %for.body, label %for.end
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; CHECK: call void @f
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; CHECK-NOT: br i1
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; CHECK: call void @f
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for.end:
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ret void
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}
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