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[CVP] Replace incoming values from unreachable blocks with undef.

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This is an attempt of fixing PR35807.
Due to the non-standard definition of dominance in LLVM, where uses in
unreachable blocks are dominated by anything, you can have, in an
unreachable block:

  %patatino = OP1 %patatino, CONSTANT

When `SimplifyInstruction` receives a PHI where an incoming value is of
the aforementioned form, in some cases, loops indefinitely.

What I propose here instead is keeping track of the incoming values
from unreachable blocks, and replacing them with undef. It fixes this
case, and it seems to be good regardless (even if we can't prove that
the value is constant, as it's coming from an unreachable block, we
can ignore it).

Differential Revision:  https://reviews.llvm.org/D41812

llvm-svn: 322006
This commit is contained in:
Davide Italiano 2018-01-08 16:34:06 +00:00
parent 1914213a11
commit d4d99b33ee
2 changed files with 89 additions and 4 deletions
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28
lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
View File

@ -14,6 +14,7 @@
#include "llvm/Transforms/Scalar/CorrelatedValuePropagation.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/Optional.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/GlobalsModRef.h"
@ -120,8 +121,8 @@ static bool processSelect(SelectInst *S, LazyValueInfo *LVI) {
return true;
}
static bool processPHI(PHINode *P, LazyValueInfo *LVI,
const SimplifyQuery &SQ) {
static bool processPHI(PHINode *P, LazyValueInfo *LVI, const SimplifyQuery &SQ,
DenseSet<BasicBlock *> &ReachableBlocks) {
bool Changed = false;
BasicBlock *BB = P->getParent();
@ -129,7 +130,18 @@ static bool processPHI(PHINode *P, LazyValueInfo *LVI,
Value *Incoming = P->getIncomingValue(i);
if (isa<Constant>(Incoming)) continue;
Value *V = LVI->getConstantOnEdge(Incoming, P->getIncomingBlock(i), BB, P);
// If the incoming value is coming from an unreachable block, replace
// it with undef and go on. This is good for two reasons:
// 1) We skip an LVI query for an unreachable block
// 2) We transform the incoming value so that the code below doesn't
// mess around with IR in unreachable blocks.
BasicBlock *IncomingBB = P->getIncomingBlock(i);
if (!ReachableBlocks.count(IncomingBB)) {
P->setIncomingValue(i, UndefValue::get(P->getType()));
continue;
}
Value *V = LVI->getConstantOnEdge(Incoming, IncomingBB, BB, P);
// Look if the incoming value is a select with a scalar condition for which
// LVI can tells us the value. In that case replace the incoming value with
@ -561,6 +573,14 @@ static Constant *getConstantAt(Value *V, Instruction *At, LazyValueInfo *LVI) {
static bool runImpl(Function &F, LazyValueInfo *LVI, const SimplifyQuery &SQ) {
bool FnChanged = false;
// Compute reachability from the entry block of this function via an RPO
// walk. We use this information when processing PHIs.
DenseSet<BasicBlock *> ReachableBlocks;
ReversePostOrderTraversal<Function *> RPOT(&F);
for (BasicBlock *BB : RPOT)
ReachableBlocks.insert(BB);
// Visiting in a pre-order depth-first traversal causes us to simplify early
// blocks before querying later blocks (which require us to analyze early
// blocks). Eagerly simplifying shallow blocks means there is strictly less
@ -575,7 +595,7 @@ static bool runImpl(Function &F, LazyValueInfo *LVI, const SimplifyQuery &SQ) {
BBChanged |= processSelect(cast<SelectInst>(II), LVI);
break;
case Instruction::PHI:
BBChanged |= processPHI(cast<PHINode>(II), LVI, SQ);
BBChanged |= processPHI(cast<PHINode>(II), LVI, SQ, ReachableBlocks);
break;
case Instruction::ICmp:
case Instruction::FCmp:

65
test/Transforms/CorrelatedValuePropagation/pr35807.ll Normal file
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@ -0,0 +1,65 @@
; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
; RUN: opt -correlated-propagation -S %s | FileCheck %s
target triple = "x86_64-apple-darwin17.4.0"
define void @patatino() {
; CHECK-LABEL: @patatino(
; CHECK-NEXT: br i1 undef, label [[BB3:%.*]], label [[BB4:%.*]]
; CHECK: bb3:
; CHECK-NEXT: br label [[BB3]]
; CHECK: bb4:
; CHECK-NEXT: br i1 undef, label [[BB40:%.*]], label [[BB22:%.*]]
; CHECK: bb7:
; CHECK-NEXT: br label [[BB14:%.*]]
; CHECK: bb12:
; CHECK-NEXT: br label [[BB14]]
; CHECK: bb14:
; CHECK-NEXT: [[TMP19:%.*]] = icmp sgt i32 undef, undef
; CHECK-NEXT: [[TMP20:%.*]] = select i1 [[TMP19]], i64 [[TMP20]], i64 0
; CHECK-NEXT: br i1 undef, label [[BB40]], label [[BB7:%.*]]
; CHECK: bb22:
; CHECK-NEXT: br label [[BB24:%.*]]
; CHECK: bb24:
; CHECK-NEXT: br label [[BB32:%.*]]
; CHECK: bb32:
; CHECK-NEXT: br i1 undef, label [[BB40]], label [[BB24]]
; CHECK: bb40:
; CHECK-NEXT: ret void
;
br i1 undef, label %bb3, label %bb4
bb3:
br label %bb3
bb4:
br i1 undef, label %bb40, label %bb22
bb7:
br label %bb14
bb12:
br label %bb14
; This block is unreachable. Due to the non-standard definition of
; dominance in LLVM where uses in unreachable blocks are dominated
; by anything, it contains an instruction of the form
; %def = OP %def, %something
bb14:
%tmp19 = icmp sgt i32 undef, undef
%tmp20 = select i1 %tmp19, i64 %tmp20, i64 0
br i1 undef, label %bb40, label %bb7
bb22:
br label %bb24
bb24:
br label %bb32
bb32:
br i1 undef, label %bb40, label %bb24
bb40:
%tmp41 = phi i64 [ 4, %bb4 ], [ %tmp20, %bb14 ], [ undef, %bb32 ]
ret void
}