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[SCEV] Make SCEV or modeling more aggressive. 

Use haveNoCommonBitsSet to figure out whether an "or" instruction
is equivalent to addition. This handles more cases than just
checking for a constant on the RHS.

Differential Revision: https://reviews.llvm.org/D32239

llvm-svn: 300746
This commit is contained in:
Eli Friedman 2017-04-19 20:19:58 +00:00
parent b45905c5a9
commit e77d2b86b4
2 changed files with 44 additions and 22 deletions
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28
llvm/lib/Analysis/ScalarEvolution.cpp
View File

@ -5328,28 +5328,12 @@ const SCEV *ScalarEvolution::createSCEV(Value *V) {
break;
case Instruction::Or:
// If the RHS of the Or is a constant, we may have something like:
// X*4+1 which got turned into X*4|1. Handle this as an Add so loop
// optimizations will transparently handle this case.
//
// In order for this transformation to be safe, the LHS must be of the
// form X*(2^n) and the Or constant must be less than 2^n.
if (ConstantInt *CI = dyn_cast<ConstantInt>(BO->RHS)) {
const SCEV *LHS = getSCEV(BO->LHS);
const APInt &CIVal = CI->getValue();
if (GetMinTrailingZeros(LHS) >=
(CIVal.getBitWidth() - CIVal.countLeadingZeros())) {
// Build a plain add SCEV.
const SCEV *S = getAddExpr(LHS, getSCEV(CI));
// If the LHS of the add was an addrec and it has no-wrap flags,
// transfer the no-wrap flags, since an or won't introduce a wrap.
if (const SCEVAddRecExpr *NewAR = dyn_cast<SCEVAddRecExpr>(S)) {
const SCEVAddRecExpr *OldAR = cast<SCEVAddRecExpr>(LHS);
const_cast<SCEVAddRecExpr *>(NewAR)->setNoWrapFlags(
OldAR->getNoWrapFlags());
}
return S;
}
// Use ValueTracking to check whether this is actually an add.
if (haveNoCommonBitsSet(BO->LHS, BO->RHS, getDataLayout(), &AC,
nullptr, &DT)) {
// There aren't any common bits set, so the add can't wrap.
auto Flags = SCEV::NoWrapFlags(SCEV::FlagNUW | SCEV::FlagNSW);
return getAddExpr(getSCEV(BO->LHS), getSCEV(BO->RHS), Flags);
}
break;

38
llvm/test/Analysis/ScalarEvolution/or-as-add.ll Normal file
View File

@ -0,0 +1,38 @@
; RUN: opt < %s -analyze -scalar-evolution | FileCheck %s
declare void @z(i32)
declare void @z2(i64)
define void @fun(i1 %bool, i32 %x) {
entry:
br label %body
body:
%i = phi i32 [ 0, %entry ], [ %i.next, %body ]
%bottom_zero = mul i32 %i, 2
%a = or i32 %bottom_zero, 1
call void @z(i32 %a)
%bool_ext = zext i1 %bool to i32
%b = or i32 %bool_ext, %bottom_zero
call void @z(i32 %b)
%shifted = lshr i32 %x, 31
%c = or i32 %shifted, %bottom_zero
call void @z(i32 %c)
%i_ext = zext i32 %i to i64
%d = or i64 %i_ext, 4294967296
call void @z2(i64 %d)
%i.next = add i32 %i, 1
%cond = icmp eq i32 %i.next, 10
br i1 %cond, label %exit, label %body
exit:
ret void
}
; CHECK: %a = or i32 %bottom_zero, 1
; CHECK-NEXT: --> {1,+,2}<%body>
; CHECK: %b = or i32 %bool_ext, %bottom_zero
; CHECK-NEXT: --> {(zext i1 %bool to i32),+,2}
; CHECK: %c = or i32 %shifted, %bottom_zero
; CHECK-NEXT: --> {(%x /u -2147483648),+,2}<%body>
; CHECK: %d = or i64 %i_ext, 4294967296
; CHECK-NEXT: --> {4294967296,+,1}<nuw><nsw><%body>