maxjhandsome
/
llvm-project
forked from OSchip/llvm-project
1
0
Fork 0
Code Issues Pull Requests Packages Releases Wiki Activity

Teach getSCEVAtScope how to handle loop phis w/invariant operands in loops w/taken backedges 

This patch really contains two pieces:
    Teach SCEV how to fold a phi in the header of a loop to the value on the backedge when a) the backedge is known to execute at least once, and b) the value is safe to use globally within the scope dominated by the original phi.
    Teach IndVarSimplify's rewriteLoopExitValues to allow loop invariant expressions which already exist (and thus don't need new computation inserted) even in loops where we can't optimize away other uses.

Differential Revision: https://reviews.llvm.org/D63224

llvm-svn: 363619
This commit is contained in:
Philip Reames 2019-06-17 21:06:17 +00:00
parent 496f77f3d3
commit 44475363e8
4 changed files with 69 additions and 43 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

44
llvm/lib/Analysis/ScalarEvolution.cpp
View File

@ -8122,27 +8122,35 @@ const SCEV *ScalarEvolution::computeSCEVAtScope(const SCEV *V, const Loop *L) {
// count. If so, we may be able to force computation of the exit
// value.
const SCEV *BackedgeTakenCount = getBackedgeTakenCount(LI);
if (const SCEVConstant *BTCC =
dyn_cast<SCEVConstant>(BackedgeTakenCount)) {
// This trivial case can show up in some degenerate cases where
// the incoming IR has not yet been fully simplified.
if (BTCC->getValue()->isZero()) {
Value *InitValue = nullptr;
bool MultipleInitValues = false;
for (unsigned i = 0; i < PN->getNumIncomingValues(); i++) {
if (!LI->contains(PN->getIncomingBlock(i))) {
if (!InitValue)
InitValue = PN->getIncomingValue(i);
else if (InitValue != PN->getIncomingValue(i)) {
MultipleInitValues = true;
break;
}
// This trivial case can show up in some degenerate cases where
// the incoming IR has not yet been fully simplified.
if (BackedgeTakenCount->isZero()) {
Value *InitValue = nullptr;
bool MultipleInitValues = false;
for (unsigned i = 0; i < PN->getNumIncomingValues(); i++) {
if (!LI->contains(PN->getIncomingBlock(i))) {
if (!InitValue)
InitValue = PN->getIncomingValue(i);
else if (InitValue != PN->getIncomingValue(i)) {
MultipleInitValues = true;
break;
}
}
if (!MultipleInitValues && InitValue)
return getSCEV(InitValue);
}
if (!MultipleInitValues && InitValue)
return getSCEV(InitValue);
}
// Do we have a loop invariant value flowing around the backedge
// for a loop which must execute the backedge?
if (!isa<SCEVCouldNotCompute>(BackedgeTakenCount) &&
isKnownPositive(BackedgeTakenCount) &&
PN->getNumIncomingValues() == 2) {
unsigned InLoopPred = LI->contains(PN->getIncomingBlock(0)) ? 0 : 1;
const SCEV *OnBackedge = getSCEV(PN->getIncomingValue(InLoopPred));
if (IsAvailableOnEntry(LI, DT, OnBackedge, PN->getParent()))
return OnBackedge;
}
if (auto *BTCC = dyn_cast<SCEVConstant>(BackedgeTakenCount)) {
// Okay, we know how many times the containing loop executes. If
// this is a constant evolving PHI node, get the final value at
// the specified iteration number.

7
llvm/lib/Transforms/Scalar/IndVarSimplify.cpp
View File

@ -635,9 +635,12 @@ bool IndVarSimplify::rewriteLoopExitValues(Loop *L, SCEVExpander &Rewriter) {
// Computing the value outside of the loop brings no benefit if it is
// definitely used inside the loop in a way which can not be optimized
// away.
// away. Avoid doing so unless we know we have a value which computes
// the ExitValue already. TODO: This should be merged into SCEV
// expander to leverage its knowledge of existing expressions.
if (ReplaceExitValue != AlwaysRepl &&
!isa<SCEVConstant>(ExitValue) && hasHardUserWithinLoop(L, Inst))
!isa<SCEVConstant>(ExitValue) && !isa<SCEVUnknown>(ExitValue) &&
hasHardUserWithinLoop(L, Inst))
continue;
bool HighCost = Rewriter.isHighCostExpansion(ExitValue, L, Inst);

57
llvm/test/Transforms/IndVarSimplify/exit_value_tests.ll
View File

@ -153,16 +153,7 @@ loopexit: ; preds = %loop
define i32 @unroll_phi_select_constant_nonzero(i32 %arg1, i32 %arg2) {
; CHECK-LABEL: @unroll_phi_select_constant_nonzero(
; CHECK-NEXT: entry:
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[I:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[I_NEXT:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[SELECTOR:%.*]] = phi i32 [ [[ARG1:%.*]], [[ENTRY]] ], [ [[ARG2:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[I_NEXT]] = add nuw nsw i32 [[I]], 1
; CHECK-NEXT: [[C:%.*]] = icmp ult i32 [[I]], 4
; CHECK-NEXT: br i1 [[C]], label [[LOOP]], label [[LOOPEXIT:%.*]]
; CHECK: loopexit:
; CHECK-NEXT: [[SELECTOR_LCSSA:%.*]] = phi i32 [ [[SELECTOR]], [[LOOP]] ]
; CHECK-NEXT: ret i32 [[SELECTOR_LCSSA]]
; CHECK-NEXT: ret i32 [[ARG2:%.*]]
;
entry:
br label %loop
@ -178,6 +169,41 @@ loopexit:
ret i32 %selector
}
declare i32 @f()
; After LCSSA formation, there's no LCSSA phi for %f since it isn't directly
; used outside the loop, and thus we can't directly replace %selector w/ %f.
define i32 @neg_unroll_phi_select_constant_nonzero(i32 %arg) {
; CHECK-LABEL: @neg_unroll_phi_select_constant_nonzero(
; CHECK-NEXT: entry:
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[I:%.*]] = phi i32 [ 0, [[ENTRY:%.*]] ], [ [[I_NEXT:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[SELECTOR:%.*]] = phi i32 [ [[ARG:%.*]], [[ENTRY]] ], [ [[F:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[F]] = call i32 @f()
; CHECK-NEXT: [[I_NEXT]] = add nuw nsw i32 [[I]], 1
; CHECK-NEXT: [[C:%.*]] = icmp ult i32 [[I]], 4
; CHECK-NEXT: br i1 [[C]], label [[LOOP]], label [[LOOPEXIT:%.*]]
; CHECK: loopexit:
; CHECK-NEXT: [[SELECTOR_LCSSA:%.*]] = phi i32 [ [[SELECTOR]], [[LOOP]] ]
; CHECK-NEXT: ret i32 [[SELECTOR_LCSSA]]
;
entry:
br label %loop
loop:
%i = phi i32 [ 0, %entry ], [ %i.next, %loop ]
%selector = phi i32 [%arg, %entry], [%f, %loop]
%f = call i32 @f()
%i.next = add nsw nuw i32 %i, 1
%c = icmp ult i32 %i, 4
br i1 %c, label %loop, label %loopexit
loopexit:
ret i32 %selector
}
define i32 @unroll_phi_select_constant_zero(i32 %arg1, i32 %arg2) {
; CHECK-LABEL: @unroll_phi_select_constant_zero(
; CHECK-NEXT: entry:
@ -201,16 +227,7 @@ define i32 @unroll_phi_select(i32 %arg1, i32 %arg2, i16 %len) {
; CHECK-LABEL: @unroll_phi_select(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[LENGTH:%.*]] = zext i16 [[LEN:%.*]] to i32
; CHECK-NEXT: br label [[LOOP:%.*]]
; CHECK: loop:
; CHECK-NEXT: [[I:%.*]] = phi i32 [ -1, [[ENTRY:%.*]] ], [ [[I_NEXT:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[SELECTOR:%.*]] = phi i32 [ [[ARG1:%.*]], [[ENTRY]] ], [ [[ARG2:%.*]], [[LOOP]] ]
; CHECK-NEXT: [[I_NEXT]] = add nsw i32 [[I]], 1
; CHECK-NEXT: [[C:%.*]] = icmp slt i32 [[I]], [[LENGTH]]
; CHECK-NEXT: br i1 [[C]], label [[LOOP]], label [[LOOPEXIT:%.*]]
; CHECK: loopexit:
; CHECK-NEXT: [[SELECTOR_LCSSA:%.*]] = phi i32 [ [[SELECTOR]], [[LOOP]] ]
; CHECK-NEXT: ret i32 [[SELECTOR_LCSSA]]
; CHECK-NEXT: ret i32 [[ARG2:%.*]]
;
entry:
%length = zext i16 %len to i32

4
llvm/test/Transforms/IndVarSimplify/pr39673.ll
View File

@ -58,15 +58,13 @@ define i16 @dom_argument(i16 %arg1, i16 %arg2) {
; CHECK-NEXT: br label [[LOOP1:%.*]]
; CHECK: loop1:
; CHECK-NEXT: [[L1:%.*]] = phi i16 [ 0, [[ENTRY:%.*]] ], [ [[L1_ADD:%.*]], [[LOOP1]] ]
; CHECK-NEXT: [[SELECTOR:%.*]] = phi i16 [ [[ARG1:%.*]], [[ENTRY]] ], [ [[ARG2:%.*]], [[LOOP1]] ]
; CHECK-NEXT: [[L1_ADD]] = add nuw nsw i16 [[L1]], 1
; CHECK-NEXT: [[CMP1:%.*]] = icmp ult i16 [[L1_ADD]], 2
; CHECK-NEXT: br i1 [[CMP1]], label [[LOOP1]], label [[LOOP2_PREHEADER:%.*]]
; CHECK: loop2.preheader:
; CHECK-NEXT: [[K1_ADD_LCSSA:%.*]] = phi i16 [ [[SELECTOR]], [[LOOP1]] ]
; CHECK-NEXT: br label [[LOOP2:%.*]]
; CHECK: loop2:
; CHECK-NEXT: [[K2:%.*]] = phi i16 [ [[K2_ADD:%.*]], [[LOOP2]] ], [ [[K1_ADD_LCSSA]], [[LOOP2_PREHEADER]] ]
; CHECK-NEXT: [[K2:%.*]] = phi i16 [ [[K2_ADD:%.*]], [[LOOP2]] ], [ [[ARG2:%.*]], [[LOOP2_PREHEADER]] ]
; CHECK-NEXT: [[L2:%.*]] = phi i16 [ [[L2_ADD:%.*]], [[LOOP2]] ], [ 0, [[LOOP2_PREHEADER]] ]
; CHECK-NEXT: [[L2_ADD]] = add nuw nsw i16 [[L2]], 1
; CHECK-NEXT: tail call void @foo(i16 [[K2]])