[llvm] r273608 - InstCombine rule to fold trunc when value available
Anna Thomas via llvm-commits
llvm-commits at lists.llvm.org
Thu Jun 23 13:22:22 PDT 2016
Author: annat
Date: Thu Jun 23 15:22:22 2016
New Revision: 273608
URL: http://llvm.org/viewvc/llvm-project?rev=273608&view=rev
Log:
InstCombine rule to fold trunc when value available
Summary:
This instcombine rule folds away trunc operations that have value available from a prior load or store.
This kind of code can be generated as a result of GVN widening the load or from source code as well.
Reviewers: reames, majnemer, sanjoy
Subscribers: llvm-commits
Differential Revision: http://reviews.llvm.org/D21246
Modified:
llvm/trunk/include/llvm/Analysis/Loads.h
llvm/trunk/lib/Analysis/Loads.cpp
llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp
llvm/trunk/test/Transforms/InstCombine/trunc.ll
Modified: llvm/trunk/include/llvm/Analysis/Loads.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/include/llvm/Analysis/Loads.h?rev=273608&r1=273607&r2=273608&view=diff
==============================================================================
--- llvm/trunk/include/llvm/Analysis/Loads.h (original)
+++ llvm/trunk/include/llvm/Analysis/Loads.h Thu Jun 23 15:22:22 2016
@@ -61,6 +61,38 @@ bool isSafeToLoadUnconditionally(Value *
/// to scan in the block, used by FindAvailableLoadedValue().
extern cl::opt<unsigned> DefMaxInstsToScan;
+/// Scan the ScanBB block backwards checking to see if we have the value at
+/// the memory address \p Ptr of type \p AccessTy locally available within a
+/// small number of instructions. If the value is available, return it.
+///
+/// If not, return the iterator for the last validated instruction that the
+/// value would be live through. If we scanned the entire block and didn't
+/// find something that invalidates *Ptr or provides it, ScanFrom would be
+/// left at begin() and this returns null. ScanFrom could also be left
+///
+/// MaxInstsToScan specifies the maximum instructions to scan in the block.
+/// If it is set to 0, it will scan the whole block. You can also optionally
+/// specify an alias analysis implementation, which makes this more precise.
+///
+/// If AATags is non-null and a load or store is found, the AA tags from the
+/// load or store are recorded there. If there are no AA tags or if no access
+/// is found, it is left unmodified.
+///
+/// IsAtomicMemOp specifies the atomicity of the memory operation that accesses
+/// \p *Ptr. We verify atomicity constraints are satisfied when value forwarding
+/// from another memory operation that has value \p *Ptr available.
+///
+/// Note that we assume the \p *Ptr is accessed through a non-volatile but
+/// potentially atomic load. Any other constraints should be verified at the
+/// caller.
+Value *FindAvailableLoadedValue(Value *Ptr, Type *AccessTy, bool IsAtomicMemOp,
+ BasicBlock *ScanBB,
+ BasicBlock::iterator &ScanFrom,
+ unsigned MaxInstsToScan,
+ AliasAnalysis *AA = nullptr,
+ AAMDNodes *AATags = nullptr,
+ bool *IsLoadCSE = nullptr);
+
/// FindAvailableLoadedValue - Scan the ScanBB block backwards (starting at
/// the instruction before ScanFrom) checking to see if we have the value at
/// the memory address *Ptr locally available within a small number of
Modified: llvm/trunk/lib/Analysis/Loads.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/Loads.cpp?rev=273608&r1=273607&r2=273608&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/Loads.cpp (original)
+++ llvm/trunk/lib/Analysis/Loads.cpp Thu Jun 23 15:22:22 2016
@@ -300,8 +300,9 @@ llvm::DefMaxInstsToScan("available-load-
"to scan backward from a given instruction, when searching for "
"available loaded value"));
-/// \brief Scan the ScanBB block backwards to see if we have the value at the
-/// memory address *Ptr locally available within a small number of instructions.
+/// Scan the ScanBB block backwards checking to see if we have the value at
+/// the memory address \p Ptr of type \p AccessTy locally available within a
+/// small number of instructions. If the value is available, return it.
///
/// The scan starts from \c ScanFrom. \c MaxInstsToScan specifies the maximum
/// instructions to scan in the block. If it is set to \c 0, it will scan the whole
@@ -318,26 +319,25 @@ llvm::DefMaxInstsToScan("available-load-
///
/// If \c AATags is non-null and a load or store is found, the AA tags from the
/// load or store are recorded there. If there are no AA tags or if no access is
-/// found, it is left unmodified.
-Value *llvm::FindAvailableLoadedValue(LoadInst *Load, BasicBlock *ScanBB,
+/// is found, it is left unmodified.
+///
+/// IsAtomicMemOp specifies the atomicity of the memory operation that accesses
+/// \p *Ptr. We verify atomicity constraints are satisfied when value forwarding
+/// from another memory operation that has value \p *Ptr available.
+///
+/// Note that we assume the \p *Ptr is accessed through a non-volatile but
+/// potentially atomic load. Any other constraints should be verified at the
+/// caller.
+Value *llvm::FindAvailableLoadedValue(Value *Ptr, Type *AccessTy, bool IsAtomicMemOp,
+ BasicBlock *ScanBB,
BasicBlock::iterator &ScanFrom,
unsigned MaxInstsToScan,
AliasAnalysis *AA, AAMDNodes *AATags,
bool *IsLoadCSE) {
+
if (MaxInstsToScan == 0)
MaxInstsToScan = ~0U;
- Value *Ptr = Load->getPointerOperand();
- Type *AccessTy = Load->getType();
-
- // We can never remove a volatile load
- if (Load->isVolatile())
- return nullptr;
-
- // Anything stronger than unordered is currently unimplemented.
- if (!Load->isUnordered())
- return nullptr;
-
const DataLayout &DL = ScanBB->getModule()->getDataLayout();
// Try to get the store size for the type.
@@ -363,14 +363,14 @@ Value *llvm::FindAvailableLoadedValue(Lo
// If this is a load of Ptr, the loaded value is available.
// (This is true even if the load is volatile or atomic, although
// those cases are unlikely.)
- if (LoadInst *LI = dyn_cast<LoadInst>(Inst))
- if (AreEquivalentAddressValues(
- LI->getPointerOperand()->stripPointerCasts(), StrippedPtr) &&
+ if (LoadInst *LI = dyn_cast<LoadInst>(Inst)) {
+ Value *LoadPtr = LI->getPointerOperand()->stripPointerCasts();
+ if (AreEquivalentAddressValues(LoadPtr, StrippedPtr) &&
CastInst::isBitOrNoopPointerCastable(LI->getType(), AccessTy, DL)) {
// We can value forward from an atomic to a non-atomic, but not the
// other way around.
- if (LI->isAtomic() < Load->isAtomic())
+ if (LI->isAtomic() < IsAtomicMemOp)
return nullptr;
if (AATags)
@@ -380,6 +380,8 @@ Value *llvm::FindAvailableLoadedValue(Lo
return LI;
}
+ }
+
if (StoreInst *SI = dyn_cast<StoreInst>(Inst)) {
Value *StorePtr = SI->getPointerOperand()->stripPointerCasts();
// If this is a store through Ptr, the value is available!
@@ -391,7 +393,7 @@ Value *llvm::FindAvailableLoadedValue(Lo
// We can value forward from an atomic to a non-atomic, but not the
// other way around.
- if (SI->isAtomic() < Load->isAtomic())
+ if (SI->isAtomic() < IsAtomicMemOp)
return nullptr;
if (AATags)
@@ -434,4 +436,44 @@ Value *llvm::FindAvailableLoadedValue(Lo
// Got to the start of the block, we didn't find it, but are done for this
// block.
return nullptr;
+
+}
+
+/// \brief Scan the ScanBB block backwards to see if we have the value at the
+/// memory address *Ptr locally available within a small number of instructions.
+///
+/// The scan starts from \c ScanFrom. \c MaxInstsToScan specifies the maximum
+/// instructions to scan in the block. If it is set to \c 0, it will scan the whole
+/// block.
+///
+/// If the value is available, this function returns it. If not, it returns the
+/// iterator for the last validated instruction that the value would be live
+/// through. If we scanned the entire block and didn't find something that
+/// invalidates \c *Ptr or provides it, \c ScanFrom is left at the last
+/// instruction processed and this returns null.
+///
+/// You can also optionally specify an alias analysis implementation, which
+/// makes this more precise.
+///
+/// If \c AATags is non-null and a load or store is found, the AA tags from the
+/// load or store are recorded there. If there are no AA tags or if no access is
+/// is found, it is left unmodified.
+Value *llvm::FindAvailableLoadedValue(LoadInst *Load, BasicBlock *ScanBB,
+ BasicBlock::iterator &ScanFrom,
+ unsigned MaxInstsToScan,
+ AliasAnalysis *AA, AAMDNodes *AATags,
+ bool *IsLoadCSE) {
+
+ // We can never remove a volatile load
+ if (Load->isVolatile())
+ return nullptr;
+
+ // Anything stronger than unordered is currently unimplemented.
+ if (!Load->isUnordered())
+ return nullptr;
+
+ // Return the full value of the load if available.
+ return FindAvailableLoadedValue(Load->getPointerOperand(), Load->getType(),
+ Load->isAtomic(), ScanBB, ScanFrom,
+ MaxInstsToScan, AA, AATags, IsLoadCSE);
}
Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp?rev=273608&r1=273607&r2=273608&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineCasts.cpp Thu Jun 23 15:22:22 2016
@@ -14,9 +14,10 @@
#include "InstCombineInternal.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/Analysis/ConstantFolding.h"
+#include "llvm/Analysis/Loads.h"
+#include "llvm/Analysis/TargetLibraryInfo.h"
#include "llvm/IR/DataLayout.h"
#include "llvm/IR/PatternMatch.h"
-#include "llvm/Analysis/TargetLibraryInfo.h"
using namespace llvm;
using namespace PatternMatch;
@@ -576,6 +577,24 @@ Instruction *InstCombiner::visitTrunc(Tr
if (Instruction *I = foldVecTruncToExtElt(CI, *this, DL))
return I;
+ // When trunc operand is a widened load, see if we can get the value from a
+ // previous store/load
+ if (auto *LI = dyn_cast<LoadInst>(Src)) {
+ BasicBlock::iterator BBI(*LI);
+
+ // Scan a few instructions up from LI and if we find a partial load/store
+ // of Type DestTy that feeds into LI, we can replace all uses of the trunc
+ // with the load/store value.
+ // This replacement can be done only in the case of non-volatile loads. If
+ // the load is atomic, its only use should be the trunc instruction. We
+ // don't want to allow other users of LI to see a value that is out of sync
+ // with the value we're folding the trunc to (in case of a race).
+ if (!LI->isVolatile() && (!LI->isAtomic() || LI->hasOneUse()))
+ if (Value *AvailableVal = FindAvailableLoadedValue(
+ LI->getPointerOperand(), DestTy, LI->isAtomic(), LI->getParent(),
+ BBI, DefMaxInstsToScan))
+ return replaceInstUsesWith(CI, AvailableVal);
+ }
return nullptr;
}
Modified: llvm/trunk/test/Transforms/InstCombine/trunc.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/trunc.ll?rev=273608&r1=273607&r2=273608&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/trunc.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/trunc.ll Thu Jun 23 15:22:22 2016
@@ -181,3 +181,60 @@ bb1:
bb2:
unreachable
}
+
+declare void @consume(i8) readonly
+define i1 @trunc_load_store(i8* align 2 %a) {
+ store i8 0, i8 *%a, align 2
+ %bca = bitcast i8* %a to i16*
+ %wide.load = load i16, i16* %bca, align 2
+ %lowhalf.1 = trunc i16 %wide.load to i8
+ call void @consume(i8 %lowhalf.1)
+ %cmp.2 = icmp ult i16 %wide.load, 256
+ ret i1 %cmp.2
+; CHECK-LABEL: @trunc_load_store
+; CHECK-NOT: trunc
+; CHECK: call void @consume(i8 0)
+}
+
+
+; The trunc can be replaced with the load value.
+define i1 @trunc_load_load(i8* align 2 %a) {
+ %pload = load i8, i8* %a, align 2
+ %bca = bitcast i8* %a to i16*
+ %wide.load = load i16, i16* %bca, align 2
+ %lowhalf = trunc i16 %wide.load to i8
+ call void @consume(i8 %lowhalf)
+ call void @consume(i8 %pload)
+ %cmp.2 = icmp ult i16 %wide.load, 256
+ ret i1 %cmp.2
+; CHECK-LABEL: @trunc_load_load
+; CHECK-NOT: trunc
+}
+
+; trunc should not be replaced since atomic load %wide.load has more than one use.
+; different values can be seen by the uses of %wide.load in case of race.
+define i1 @trunc_atomic_loads(i8* align 2 %a) {
+ %pload = load atomic i8, i8* %a unordered, align 2
+ %bca = bitcast i8* %a to i16*
+ %wide.load = load atomic i16, i16* %bca unordered, align 2
+ %lowhalf = trunc i16 %wide.load to i8
+ call void @consume(i8 %lowhalf)
+ call void @consume(i8 %pload)
+ %cmp.2 = icmp ult i16 %wide.load, 256
+ ret i1 %cmp.2
+; CHECK-LABEL: @trunc_atomic_loads
+; CHECK: trunc
+}
+
+; trunc cannot be replaced since store size is not trunc result size
+define i1 @trunc_different_size_load(i16 * align 2 %a) {
+ store i16 0, i16 *%a, align 2
+ %bca = bitcast i16* %a to i32*
+ %wide.load = load i32, i32* %bca, align 2
+ %lowhalf = trunc i32 %wide.load to i8
+ call void @consume(i8 %lowhalf)
+ %cmp.2 = icmp ult i32 %wide.load, 256
+ ret i1 %cmp.2
+; CHECK-LABEL: @trunc_different_size_load
+; CHECK: %lowhalf = trunc i32 %wide.load to i8
+}
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