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Make Value::isDereferenceablePointer handle offsets to pointer types with dereferenceable attributes

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When we have a parameter (or call site return) with a dereferenceable
attribute, it can specify the size of an array pointed to by that parameter. If
we have a value for which we can accumulate a constant offset to such a
parameter, then we can use that offset in a direct comparison with the size
specified by the dereferenceable attribute.

This enables us to handle cases like this:

  int foo(int a[static 3]) {
    return a[2]; /* this is always dereferenceable */
  }

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213447 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Hal Finkel 2014-07-19 03:25:16 +00:00
parent b0a5225e6f
commit 7c11695a23
2 changed files with 103 additions and 0 deletions
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21
lib/IR/Value.cpp
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@ -574,6 +574,27 @@ static bool isDereferenceablePointer(const Value *V, const DataLayout *DL,
/// isDereferenceablePointer - Test if this value is always a pointer to
/// allocated and suitably aligned memory for a simple load or store.
bool Value::isDereferenceablePointer(const DataLayout *DL) const {
// When dereferenceability information is provided by a dereferenceable
// attribute, we know exactly how many bytes are dereferenceable. If we can
// determine the exact offset to the attributed variable, we can use that
// information here.
Type *Ty = getType()->getPointerElementType();
if (Ty->isSized() && DL) {
APInt Offset(DL->getTypeStoreSizeInBits(getType()), 0);
const Value *BV = stripAndAccumulateInBoundsConstantOffsets(*DL, Offset);
APInt DerefBytes(Offset.getBitWidth(), 0);
if (const Argument *A = dyn_cast<Argument>(BV))
DerefBytes = A->getDereferenceableBytes();
else if (ImmutableCallSite CS = BV)
DerefBytes = CS.getDereferenceableBytes(0);
if (DerefBytes.getBoolValue() && Offset.isNonNegative()) {
if (DerefBytes.uge(Offset + DL->getTypeStoreSize(Ty)))
return true;
}
}
SmallPtrSet<const Value *, 32> Visited;
return ::isDereferenceablePointer(this, DL, Visited);
}

82
test/Transforms/LICM/hoist-deref-load.ll
View File

@ -82,5 +82,87 @@ for.end: ; preds = %for.inc, %entry
ret void
}
; This test represents the following function:
; void test3(int * restrict a, int * restrict b, int c[static 3], int n) {
; for (int i = 0; i < n; ++i)
; if (a[i] > 0)
; a[i] = c[2]*b[i];
; }
; and we want to hoist the load of c[2] out of the loop. This can be done only
; because the dereferenceable attribute is on %c.
; CHECK-LABEL: @test3
; CHECK: load i32* %c2, align 4
; CHECK: for.body:
define void @test3(i32* noalias nocapture %a, i32* noalias nocapture readonly %b, i32* nocapture readonly dereferenceable(12) %c, i32 %n) #0 {
entry:
%cmp11 = icmp sgt i32 %n, 0
br i1 %cmp11, label %for.body, label %for.end
for.body: ; preds = %entry, %for.inc
%indvars.iv = phi i64 [ %indvars.iv.next, %for.inc ], [ 0, %entry ]
%arrayidx = getelementptr inbounds i32* %a, i64 %indvars.iv
%0 = load i32* %arrayidx, align 4
%cmp1 = icmp sgt i32 %0, 0
br i1 %cmp1, label %if.then, label %for.inc
if.then: ; preds = %for.body
%c2 = getelementptr inbounds i32* %c, i64 2
%1 = load i32* %c2, align 4
%arrayidx3 = getelementptr inbounds i32* %b, i64 %indvars.iv
%2 = load i32* %arrayidx3, align 4
%mul = mul nsw i32 %2, %1
store i32 %mul, i32* %arrayidx, align 4
br label %for.inc
for.inc: ; preds = %for.body, %if.then
%indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
%lftr.wideiv = trunc i64 %indvars.iv.next to i32
%exitcond = icmp eq i32 %lftr.wideiv, %n
br i1 %exitcond, label %for.end, label %for.body
for.end: ; preds = %for.inc, %entry
ret void
}
; This is the same as @test3, but with a dereferenceable attribute on %c with a
; size too small to cover c[2] (and so we should not hoist it).
; CHECK-LABEL: @test4
; CHECK: if.then:
; CHECK: load i32* %c2, align 4
define void @test4(i32* noalias nocapture %a, i32* noalias nocapture readonly %b, i32* nocapture readonly dereferenceable(11) %c, i32 %n) #0 {
entry:
%cmp11 = icmp sgt i32 %n, 0
br i1 %cmp11, label %for.body, label %for.end
for.body: ; preds = %entry, %for.inc
%indvars.iv = phi i64 [ %indvars.iv.next, %for.inc ], [ 0, %entry ]
%arrayidx = getelementptr inbounds i32* %a, i64 %indvars.iv
%0 = load i32* %arrayidx, align 4
%cmp1 = icmp sgt i32 %0, 0
br i1 %cmp1, label %if.then, label %for.inc
if.then: ; preds = %for.body
%c2 = getelementptr inbounds i32* %c, i64 2
%1 = load i32* %c2, align 4
%arrayidx3 = getelementptr inbounds i32* %b, i64 %indvars.iv
%2 = load i32* %arrayidx3, align 4
%mul = mul nsw i32 %2, %1
store i32 %mul, i32* %arrayidx, align 4
br label %for.inc
for.inc: ; preds = %for.body, %if.then
%indvars.iv.next = add nuw nsw i64 %indvars.iv, 1
%lftr.wideiv = trunc i64 %indvars.iv.next to i32
%exitcond = icmp eq i32 %lftr.wideiv, %n
br i1 %exitcond, label %for.end, label %for.body
for.end: ; preds = %for.inc, %entry
ret void
}
attributes #0 = { nounwind uwtable }