[PATCH] D60975: Convert a masked.gather of at most one element to a masked.load

[Philip Reames via Phabricator via llvm-commits]

reames created this revision.
reames added reviewers: craig.topper, spatel.
Herald added subscribers: llvm-commits, bollu, mcrosier.
Herald added a project: LLVM.

Split off of D59703 <https://reviews.llvm.org/D59703>.  If we have a gather with a single element, that's equivalent to a single element masked.load from the respective address.


Repository:
  rL LLVM

https://reviews.llvm.org/D60975

Files:
  lib/Transforms/InstCombine/InstCombineCalls.cpp
  test/Transforms/InstCombine/masked_intrinsics.ll


Index: test/Transforms/InstCombine/masked_intrinsics.ll
===================================================================
--- test/Transforms/InstCombine/masked_intrinsics.ll
+++ test/Transforms/InstCombine/masked_intrinsics.ll
@@ -206,8 +206,12 @@
 ; CHECK-NEXT:    [[PTRS:%.*]] = getelementptr double, double* [[BASE:%.*]], <4 x i64> <i64 undef, i64 undef, i64 2, i64 undef>
 ; CHECK-NEXT:    [[PT_V1:%.*]] = insertelement <4 x double> undef, double [[PT:%.*]], i64 0
 ; CHECK-NEXT:    [[PT_V2:%.*]] = shufflevector <4 x double> [[PT_V1]], <4 x double> undef, <4 x i32> <i32 0, i32 0, i32 undef, i32 0>
-; CHECK-NEXT:    [[RES:%.*]] = call <4 x double> @llvm.masked.gather.v4f64.v4p0f64(<4 x double*> [[PTRS]], i32 4, <4 x i1> <i1 false, i1 false, i1 true, i1 false>, <4 x double> [[PT_V2]])
-; CHECK-NEXT:    ret <4 x double> [[RES]]
+; CHECK-NEXT:    [[BC:%.*]] = bitcast <4 x double*> [[PTRS]] to <4 x <1 x double>*>
+; CHECK-NEXT:    [[TMP1:%.*]] = extractelement <4 x <1 x double>*> [[BC]], i64 2
+; CHECK-NEXT:    [[UNMASKEDLOAD:%.*]] = load <1 x double>, <1 x double>* [[TMP1]], align 4
+; CHECK-NEXT:    [[TMP2:%.*]] = shufflevector <1 x double> [[UNMASKEDLOAD]], <1 x double> undef, <4 x i32> <i32 0, i32 undef, i32 undef, i32 undef>
+; CHECK-NEXT:    [[TMP3:%.*]] = shufflevector <4 x double> [[PT_V2]], <4 x double> [[TMP2]], <4 x i32> <i32 0, i32 1, i32 4, i32 3>
+; CHECK-NEXT:    ret <4 x double> [[TMP3]]
 ;
   %ptrs = getelementptr double, double *%base, <4 x i64> <i64 0, i64 1, i64 2, i64 3>
   %pt_v1 = insertelement <4 x double> undef, double %pt, i64 0
Index: lib/Transforms/InstCombine/InstCombineCalls.cpp
===================================================================
--- lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -21,6 +21,7 @@
 #include "llvm/ADT/Statistic.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/Analysis/AssumptionCache.h"
+#include "llvm/Analysis/Loads.h"
 #include "llvm/Analysis/InstructionSimplify.h"
 #include "llvm/Analysis/MemoryBuiltins.h"
 #include "llvm/Transforms/Utils/Local.h"
@@ -1213,7 +1214,6 @@
 }
 
 // TODO, Obvious Missing Transforms:
-// * Single constant active lane load -> load
 // * Dereferenceable address & few lanes -> scalarize speculative load/selects
 // * Adjacent vector addresses -> masked.load
 // * Narrow width by halfs excluding zero/undef lanes
@@ -1222,9 +1222,37 @@
 static Instruction *simplifyMaskedGather(IntrinsicInst &II, InstCombiner &IC) {
   // If the mask is all zeros, return the "passthru" argument of the gather.
   auto *ConstMask = dyn_cast<Constant>(II.getArgOperand(2));
-  if (ConstMask && ConstMask->isNullValue())
+  if (!ConstMask)
+    return nullptr;
+  if (ConstMask->isNullValue())
     return IC.replaceInstUsesWith(II, II.getArgOperand(3));
 
+  APInt DemandedElts = possiblyDemandedEltsInMask(ConstMask);
+  // If we have at most a single lane active, emit a scalar masked.load.  If
+  // the mask is known to be active (as opposed to simply not known inactive),
+  // then the masked.load combines will convert it to a simple load.
+  if (DemandedElts.isPowerOf2()) {
+    // Note: APInt indexes the bit vector from LSB to MSB, thus
+    // countTrailingZeros returns the index in the vector which is set. 
+    unsigned Idx = DemandedElts.countTrailingZeros();
+    auto &B = IC.Builder;
+    auto *PtrLane = B.CreateExtractElement(II.getArgOperand(0), Idx);
+    unsigned Alignment = cast<ConstantInt>(II.getArgOperand(1))->getZExtValue();
+    auto *MaskLane = B.CreateExtractElement(II.getArgOperand(2), Idx);
+    auto *PassThrough = II.getArgOperand(3);
+    auto *PTLane = B.CreateExtractElement(PassThrough, Idx);
+    // TODO: pull out a scalar masked load helper function.
+    auto *PTy = PointerType::get(VectorType::get(PTLane->getType(), 1),
+                                PtrLane->getType()->getPointerAddressSpace());
+    auto *ML = B.CreateMaskedLoad(B.CreateBitCast(PtrLane, PTy),
+                                  Alignment,
+                                  B.CreateVectorSplat(1, MaskLane),
+                                  B.CreateVectorSplat(1, PTLane));
+    auto *E = B.CreateExtractElement(ML, (uint64_t)0);
+    auto *Res = B.CreateInsertElement(PassThrough, E, Idx);
+    return IC.replaceInstUsesWith(II, Res);
+  }
+
   return nullptr;
 }
 


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