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Teach SCEV's icmp simplification logic that a-b == 0 is equivalent to a == b. 

This also required making recursive simplifications until
nothing changes or a hard limit (currently 3) is hit.

With the simplification in place indvars can canonicalize
loops of the form
for (unsigned i = 0; i < a-b; ++i)
into
for (unsigned i = 0; i != a-b; ++i)
which used to fail because SCEV created a weird umax expr
for the backedge taken count.

llvm-svn: 157701
This commit is contained in:
Benjamin Kramer 2012-05-30 18:32:23 +00:00
parent 6acd46f5e9
commit 50b26ebb2b
3 changed files with 64 additions and 2 deletions
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3
llvm/include/llvm/Analysis/ScalarEvolution.h
View File

@ -837,7 +837,8 @@ namespace llvm {
///
bool SimplifyICmpOperands(ICmpInst::Predicate &Pred,
const SCEV *&LHS,
const SCEV *&RHS);
const SCEV *&RHS,
unsigned Depth = 0);
/// getLoopDisposition - Return the "disposition" of the given SCEV with
/// respect to the given loop.

21
llvm/lib/Analysis/ScalarEvolution.cpp
View File

@ -5605,9 +5605,14 @@ static bool HasSameValue(const SCEV *A, const SCEV *B) {
/// predicate Pred. Return true iff any changes were made.
///
bool ScalarEvolution::SimplifyICmpOperands(ICmpInst::Predicate &Pred,
const SCEV *&LHS, const SCEV *&RHS) {
const SCEV *&LHS, const SCEV *&RHS,
unsigned Depth) {
bool Changed = false;
// If we hit the max recursion limit bail out.
if (Depth >= 3)
return false;
// Canonicalize a constant to the right side.
if (const SCEVConstant *LHSC = dyn_cast<SCEVConstant>(LHS)) {
// Check for both operands constant.
@ -5645,6 +5650,15 @@ bool ScalarEvolution::SimplifyICmpOperands(ICmpInst::Predicate &Pred,
default: llvm_unreachable("Unexpected ICmpInst::Predicate value!");
case ICmpInst::ICMP_EQ:
case ICmpInst::ICMP_NE:
// Fold ((-1) * %a) + %b == 0 (equivalent to %b-%a == 0) into %a == %b.
if (!RA)
if (const SCEVAddExpr *AE = dyn_cast<SCEVAddExpr>(LHS))
if (const SCEVMulExpr *ME = dyn_cast<SCEVMulExpr>(AE->getOperand(0)))
if (ME->getOperand(0)->isAllOnesValue()) {
RHS = AE->getOperand(1);
LHS = ME->getOperand(1);
Changed = true;
}
break;
case ICmpInst::ICMP_UGE:
if ((RA - 1).isMinValue()) {
@ -5846,6 +5860,11 @@ bool ScalarEvolution::SimplifyICmpOperands(ICmpInst::Predicate &Pred,
// TODO: More simplifications are possible here.
// Recursively simplify until we either hit a recursion limit or nothing
// changes.
if (Changed)
return SimplifyICmpOperands(Pred, LHS, RHS, Depth+1);
return Changed;
trivially_true:

42
llvm/test/Transforms/IndVarSimplify/ult-sub-to-eq.ll Normal file
View File

@ -0,0 +1,42 @@
; RUN: opt -S -indvars < %s | FileCheck %s
define void @test1(float* nocapture %autoc, float* nocapture %data, float %d, i32 %data_len, i32 %sample) nounwind {
entry:
%sub = sub i32 %data_len, %sample
%cmp4 = icmp eq i32 %data_len, %sample
br i1 %cmp4, label %for.end, label %for.body
for.body: ; preds = %entry, %for.body
%indvars.iv = phi i64 [ %indvars.iv.next, %for.body ], [ 0, %entry ]
%0 = trunc i64 %indvars.iv to i32
%add = add i32 %0, %sample
%idxprom = zext i32 %add to i64
%arrayidx = getelementptr inbounds float* %data, i64 %idxprom
%1 = load float* %arrayidx, align 4
%mul = fmul float %1, %d
%arrayidx2 = getelementptr inbounds float* %autoc, i64 %indvars.iv
%2 = load float* %arrayidx2, align 4
%add3 = fadd float %2, %mul
store float %add3, float* %arrayidx2, align 4
%indvars.iv.next = add i64 %indvars.iv, 1
%3 = trunc i64 %indvars.iv.next to i32
%cmp = icmp ult i32 %3, %sub
br i1 %cmp, label %for.body, label %for.end
for.end: ; preds = %for.body, %entry
ret void
; CHECK: @test1
; First check that we move the sub into the preheader, it doesn't have to be
; executed if %cmp4 == false
; CHECK: for.body.preheader:
; CHECK: sub i32 %data_len, %sample
; CHECK: br label %for.body
; Second, check that we turn the IV test into an eq.
; CHECK: %lftr.wideiv = trunc i64 %indvars.iv.next to i32
; CHECK: %exitcond = icmp ne i32 %lftr.wideiv, %0
; CHECK: br i1 %exitcond, label %for.body, label %for.end.loopexit
}