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Fix mem2reg to correctly handle allocas only used in a single block

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Currently, a load from an alloca that is used in as single block and is not preceded
by a store is replaced by undef. This is not always correct if the single block is
inside a loop.
Fix the logic so that:
1) If there are no stores in the block, replace the load with an undef, as before.
2) If there is a store (regardless of where it is in the block w.r.t the load), bail
out, and let the rest of mem2reg handle this alloca.

Patch by: gil.rapaport@intel.com
Differential Revision: http://reviews.llvm.org/D11355

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@242884 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Michael Kuperstein 2015-07-22 10:29:29 +00:00
parent a5812d5bbf
commit 6418278e7d
2 changed files with 67 additions and 15 deletions
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38
lib/Transforms/Utils/PromoteMemoryToRegister.cpp
View File

@ -425,14 +425,17 @@ static bool rewriteSingleStoreAlloca(AllocaInst *AI, AllocaInfo &Info,
/// using the Alloca.
///
/// If we cannot promote this alloca (because it is read before it is written),
/// return true. This is necessary in cases where, due to control flow, the
/// alloca is potentially undefined on some control flow paths. e.g. code like
/// this is potentially correct:
///
/// for (...) { if (c) { A = undef; undef = B; } }
///
/// ... so long as A is not used before undef is set.
static void promoteSingleBlockAlloca(AllocaInst *AI, const AllocaInfo &Info,
/// return false. This is necessary in cases where, due to control flow, the
/// alloca is undefined only on some control flow paths. e.g. code like
/// this is correct in LLVM IR:
/// // A is an alloca with no stores so far
/// for (...) {
/// int t = *A;
/// if (!first_iteration)
/// use(t);
/// *A = 42;
/// }
static bool promoteSingleBlockAlloca(AllocaInst *AI, const AllocaInfo &Info,
LargeBlockInfo &LBI,
AliasSetTracker *AST) {
// The trickiest case to handle is when we have large blocks. Because of this,
@ -467,10 +470,15 @@ static void promoteSingleBlockAlloca(AllocaInst *AI, const AllocaInfo &Info,
std::make_pair(LoadIdx,
static_cast<StoreInst *>(nullptr)),
less_first());
if (I == StoresByIndex.begin())
// If there is no store before this load, the load takes the undef value.
LI->replaceAllUsesWith(UndefValue::get(LI->getType()));
if (I == StoresByIndex.begin()) {
if (StoresByIndex.empty())
// If there are no stores, the load takes the undef value.
LI->replaceAllUsesWith(UndefValue::get(LI->getType()));
else
// There is no store before this load, bail out (load may be affected
// by the following stores - see main comment).
return false;
}
else
// Otherwise, there was a store before this load, the load takes its value.
LI->replaceAllUsesWith(std::prev(I)->second->getOperand(0));
@ -506,6 +514,7 @@ static void promoteSingleBlockAlloca(AllocaInst *AI, const AllocaInfo &Info,
}
++NumLocalPromoted;
return true;
}
void PromoteMem2Reg::run() {
@ -557,9 +566,8 @@ void PromoteMem2Reg::run() {
// If the alloca is only read and written in one basic block, just perform a
// linear sweep over the block to eliminate it.
if (Info.OnlyUsedInOneBlock) {
promoteSingleBlockAlloca(AI, Info, LBI, AST);
if (Info.OnlyUsedInOneBlock &&
promoteSingleBlockAlloca(AI, Info, LBI, AST)) {
// The alloca has been processed, move on.
RemoveFromAllocasList(AllocaNum);
continue;

44
test/Transforms/Mem2Reg/pr24179.ll Normal file
View File

@ -0,0 +1,44 @@
; RUN: opt -mem2reg < %s -S | FileCheck %s
declare i32 @def(i32)
declare i1 @use(i32)
; Special case of a single-BB alloca does not apply here since the load
; is affected by the following store. Expect this case to be identified
; and a PHI node to be created.
define void @test1() {
; CHECK-LABEL: @test1(
entry:
%t = alloca i32
br label %loop
loop:
%v = load i32, i32* %t
%c = call i1 @use(i32 %v)
; CHECK: [[PHI:%.*]] = phi i32 [ undef, %entry ], [ %n, %loop ]
; CHECK: call i1 @use(i32 [[PHI]])
%n = call i32 @def(i32 7)
store i32 %n, i32* %t
br i1 %c, label %loop, label %exit
exit:
ret void
}
; Same as above, except there is no following store. The alloca should just be
; replaced with an undef
define void @test2() {
; CHECK-LABEL: @test2(
entry:
%t = alloca i32
br label %loop
loop:
%v = load i32, i32* %t
%c = call i1 @use(i32 %v)
; CHECK: %c = call i1 @use(i32 undef)
br i1 %c, label %loop, label %exit
exit:
ret void
}