InstCombine rule to fold truncs whose value is available

We can fold truncs whose operand feeds from a load, if the trunc value
is available through a prior load/store.

This change is from: http://reviews.llvm.org/D21246, which folded the
trunc but missed the bitcast or ptrtoint/inttoptr required in the RAUW
call, when the load type didnt match the prior load/store type.

Differential Revision: http://reviews.llvm.org/D21791

llvm-svn: 274853

include/llvm/Analysis/Loads.h

@@ -58,6 +58,44 @@ bool isSafeToLoadUnconditionally(Value *V, unsigned Align,
 /// to scan in the block, used by FindAvailableLoadedValue().
 extern cl::opt<unsigned> DefMaxInstsToScan;
 
+/// \brief Scan backwards to see if we have the value of type \p AccessTy
+/// at the memory address \p Ptr locally available within a
+/// small number of instructions. If the value is available, return it.
+///
+/// You can use this function to scan across multiple blocks: after you call
+/// this function, if ScanFrom points at the beginning of the block, it's safe
+/// to continue scanning the predecessors.
+/// 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.
+///
+/// \param Ptr The memory location whose contents we are retrieving
+/// \param AccessTy The type (and size) of the contents we need from \p Ptr
+/// \param 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.
+/// \param ScanBB The basic block to scan. FIXME: This is redundant.
+/// \param [in,out] ScanFrom The location to start scanning from. When this
+/// function returns, it points at the last instruction scanned.
+/// \param MaxInstsToScan The maximum number of instructions to scan. If this
+/// is zero, the whole block will be scanned.
+/// \param AA Optional pointer to alias analysis, to make the scan more
+/// precise.
+/// \param [out] AATags The aliasing metadata for the operation which produced
+/// the value. FIXME: This is basically useless.
+///
+/// \param [out] IsLoadCSE Whether the returned value is a load from the same
+/// location in memory, as opposed to the value operand of a store.
+///
+/// \returns The found value, or nullptr if no value is found.
+Value *FindAvailableLoadedValue(Value *Ptr, Type *AccessTy, bool IsAtomicMemOp,
+                                BasicBlock *ScanBB,
+                                BasicBlock::iterator &ScanFrom,
+                                unsigned MaxInstsToScan,
+                                AliasAnalysis *AA = nullptr,
+                                AAMDNodes *AATags = nullptr,
+                                bool *IsLoadCSE = nullptr);
+
 /// \brief Scan backwards to see if we have the value of the given load
 /// available locally within a small number of instructions.
 ///

lib/Analysis/Loads.cpp

@@ -297,27 +297,17 @@ llvm::DefMaxInstsToScan("available-load-scan-limit", cl::init(6), cl::Hidden,
            "to scan backward from a given instruction, when searching for "
            "available loaded value"));
 
-Value *llvm::FindAvailableLoadedValue(LoadInst *Load, BasicBlock *ScanBB,
+
+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.
   uint64_t AccessSize = DL.getTypeStoreSize(AccessTy);
 
@@ -348,7 +338,7 @@ Value *llvm::FindAvailableLoadedValue(LoadInst *Load, BasicBlock *ScanBB,
 
         // 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)
@@ -369,7 +359,7 @@ Value *llvm::FindAvailableLoadedValue(LoadInst *Load, BasicBlock *ScanBB,
 
         // 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)
@@ -413,3 +403,24 @@ Value *llvm::FindAvailableLoadedValue(LoadInst *Load, BasicBlock *ScanBB,
   // block.
   return nullptr;
 }
+
+
+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);
+}

lib/Transforms/InstCombine/InstCombineCasts.cpp

@@ -13,9 +13,10 @@
 
 #include "InstCombineInternal.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;
 
@@ -575,6 +576,27 @@ Instruction *InstCombiner::visitTrunc(TruncInst &CI) {
   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, with
+    // ordering at most unordered. 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->isUnordered() && (!LI->isAtomic() || LI->hasOneUse()))
+      if (Value *AvailableVal = FindAvailableLoadedValue(
+              LI->getPointerOperand(), DestTy, LI->isAtomic(), LI->getParent(),
+              BBI, DefMaxInstsToScan))
+        return replaceInstUsesWith(
+            CI, Builder->CreateBitOrPointerCast(AvailableVal, CI.getType(),
+                                                CI.getName() + ".cast"));
+  }
   return nullptr;
 }
 

test/Transforms/InstCombine/trunc.ll

@@ -181,3 +181,133 @@ bb1:
 bb2:
   unreachable
 }
+
+
+; the trunc can be replaced from value available from store
+; load feeding into trunc left as-is.
+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: %wide.load = load i16, i16* %bca, align 2
+; CHECK-NOT: trunc
+; CHECK: call void @consume(i8 0)
+}
+
+
+; The trunc can be replaced with the load value.
+; both loads left as-is, since they have uses.
+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-NEXT: %pload = load i8, i8* %a, align 2
+; CHECK-NEXT: %bca  = bitcast i8* %a to i16*
+; CHECK-NEXT: %wide.load = load i16, i16* %bca, align 2
+; CHECK-NEXT: call void @consume(i8 %pload)
+; CHECK-NEXT: call void @consume(i8 %pload)
+; CHECK-NEXT: %cmp.2 = icmp ult i16 %wide.load, 256
+}
+
+; Store and load to same memory location address generated through GEP.
+; trunc can be removed by using the store value.
+define void @trunc_with_gep_memaccess(i16* align 2 %p) {
+  %t0 = getelementptr i16, i16* %p, i32 1
+  store i16 2, i16* %t0
+  %t1 = getelementptr i16, i16* %p, i32 1
+  %x = load i16, i16* %t1
+  %lowhalf = trunc i16 %x to i8
+  call void @consume(i8 %lowhalf)
+  ret void
+; CHECK-LABEL: @trunc_with_gep_memaccess
+; CHECK-NOT: trunc
+; CHECK: call void @consume(i8 2)
+}
+
+; 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 can be replaced since atomic load has single use.
+; atomic load is also removed since use is removed.
+define void @trunc_atomic_single_load(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)
+  ret void
+; CHECK-LABEL: @trunc_atomic_single_load
+; CHECK-NOT: trunc
+; CHECK-NOT: %wide.load = load atomic i16, i16* %bca unordered, align 2
+}
+
+
+; trunc cannot be replaced since load's atomic ordering is higher than unordered
+define void @trunc_atomic_monotonic(i8* align 2 %a) {
+  %pload = load atomic i8, i8* %a monotonic, align 2
+  %bca  = bitcast i8* %a to i16*
+  %wide.load = load atomic i16, i16* %bca monotonic, align 2
+  %lowhalf = trunc i16 %wide.load to i8
+  call void @consume(i8 %lowhalf)
+  call void @consume(i8 %pload)
+  ret void
+; CHECK-LABEL: @trunc_atomic_monotonic
+; CHECK: %wide.load = load atomic i16, i16* %bca monotonic, align 2
+; CHECK: trunc
+}
+
+; trunc cannot be replaced since store size (i16) is not trunc result size (i8).
+; FIXME: we could get the i8 content of trunc from the i16 store value.
+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
+}
+
+declare void @consume_f(float) readonly
+; bitcast required since trunc result type and %fload are different types.
+; so replace the trunc with bitcast.
+define i1 @trunc_avoid_bitcast(float* %b) {
+  %fload = load float, float* %b
+  %bca = bitcast float* %b to i64*
+  %iload = load i64, i64* %bca
+  %low32 = trunc i64 %iload to i32
+  call void @consume_f(float %fload)
+  %cmp.2 = icmp ult i32 %low32, 256
+  ret i1 %cmp.2
+; CHECK-LABEL: @trunc_avoid_bitcast
+; CHECK-NOT: %low32 = trunc i64 %iload to i32
+; CHECK: %low32.cast = bitcast float %fload to i32
+; CHECK: %cmp.2 = icmp ult i32 %low32.cast, 256
+}