[AMDGPU] Use d16 flag for image.sample instructions

Image.sample instruction can be forced to return half type instead of
float when d16 flag is enabled.

This patch adds new pattern in InstCombine to detect if output of
image.sample is used later only by fptrunc which converts the type
from float to half. If pattern is detected then fptrunc and image.sample
are combined to single image.sample which is returning half type.
Later in Lowering part d16 flag is added to image sample intrinsic.

Differential Revision: https://reviews.llvm.org/D124232

Author: mariusz-sikora-at-amd

llvm/lib/Target/AMDGPU/AMDGPUInstCombineIntrinsic.cpp

@@ -110,33 +110,42 @@ static Value *convertTo16Bit(Value &V, InstCombiner::BuilderTy &Builder) {
   llvm_unreachable("Should never be called!");
 }
 
-/// Applies Function(II.Args, II.ArgTys) and replaces the intrinsic call with
-/// the modified arguments.
+/// Applies Func(OldIntr.Args, OldIntr.ArgTys), creates intrinsic call with
+/// modified arguments (based on OldIntr) and replaces InstToReplace with
+/// this newly created intrinsic call.
 static Optional<Instruction *> modifyIntrinsicCall(
-    IntrinsicInst &II, unsigned NewIntr, InstCombiner &IC,
+    IntrinsicInst &OldIntr, Instruction &InstToReplace, unsigned NewIntr,
+    InstCombiner &IC,
     std::function<void(SmallVectorImpl<Value *> &, SmallVectorImpl<Type *> &)>
         Func) {
   SmallVector<Type *, 4> ArgTys;
-  if (!Intrinsic::getIntrinsicSignature(II.getCalledFunction(), ArgTys))
+  if (!Intrinsic::getIntrinsicSignature(OldIntr.getCalledFunction(), ArgTys))
     return None;
 
-  SmallVector<Value *, 8> Args(II.args());
+  SmallVector<Value *, 8> Args(OldIntr.args());
 
   // Modify arguments and types
   Func(Args, ArgTys);
 
-  Function *I = Intrinsic::getDeclaration(II.getModule(), NewIntr, ArgTys);
+  Function *I = Intrinsic::getDeclaration(OldIntr.getModule(), NewIntr, ArgTys);
 
   CallInst *NewCall = IC.Builder.CreateCall(I, Args);
-  NewCall->takeName(&II);
-  NewCall->copyMetadata(II);
+  NewCall->takeName(&OldIntr);
+  NewCall->copyMetadata(OldIntr);
   if (isa<FPMathOperator>(NewCall))
-    NewCall->copyFastMathFlags(&II);
+    NewCall->copyFastMathFlags(&OldIntr);
 
   // Erase and replace uses
-  if (!II.getType()->isVoidTy())
-    IC.replaceInstUsesWith(II, NewCall);
-  return IC.eraseInstFromFunction(II);
+  if (!InstToReplace.getType()->isVoidTy())
+    IC.replaceInstUsesWith(InstToReplace, NewCall);
+
+  bool RemoveOldIntr = &OldIntr != &InstToReplace;
+
+  auto RetValue = IC.eraseInstFromFunction(InstToReplace);
+  if (RemoveOldIntr)
+    IC.eraseInstFromFunction(OldIntr);
+
+  return RetValue;
 }
 
 static Optional<Instruction *>
@@ -153,7 +162,7 @@ simplifyAMDGCNImageIntrinsic(const GCNSubtarget *ST,
             AMDGPU::getImageDimIntrinsicByBaseOpcode(LZMappingInfo->LZ,
                                                      ImageDimIntr->Dim);
         return modifyIntrinsicCall(
-            II, NewImageDimIntr->Intr, IC, [&](auto &Args, auto &ArgTys) {
+            II, II, NewImageDimIntr->Intr, IC, [&](auto &Args, auto &ArgTys) {
               Args.erase(Args.begin() + ImageDimIntr->LodIndex);
             });
       }
@@ -170,7 +179,7 @@ simplifyAMDGCNImageIntrinsic(const GCNSubtarget *ST,
             AMDGPU::getImageDimIntrinsicByBaseOpcode(MIPMappingInfo->NONMIP,
                                                      ImageDimIntr->Dim);
         return modifyIntrinsicCall(
-            II, NewImageDimIntr->Intr, IC, [&](auto &Args, auto &ArgTys) {
+            II, II, NewImageDimIntr->Intr, IC, [&](auto &Args, auto &ArgTys) {
               Args.erase(Args.begin() + ImageDimIntr->MipIndex);
             });
       }
@@ -187,7 +196,7 @@ simplifyAMDGCNImageIntrinsic(const GCNSubtarget *ST,
             AMDGPU::getImageDimIntrinsicByBaseOpcode(BiasMappingInfo->NoBias,
                                                      ImageDimIntr->Dim);
         return modifyIntrinsicCall(
-            II, NewImageDimIntr->Intr, IC, [&](auto &Args, auto &ArgTys) {
+            II, II, NewImageDimIntr->Intr, IC, [&](auto &Args, auto &ArgTys) {
               Args.erase(Args.begin() + ImageDimIntr->BiasIndex);
               ArgTys.erase(ArgTys.begin() + ImageDimIntr->BiasTyArg);
             });
@@ -205,13 +214,41 @@ simplifyAMDGCNImageIntrinsic(const GCNSubtarget *ST,
             AMDGPU::getImageDimIntrinsicByBaseOpcode(
                 OffsetMappingInfo->NoOffset, ImageDimIntr->Dim);
         return modifyIntrinsicCall(
-            II, NewImageDimIntr->Intr, IC, [&](auto &Args, auto &ArgTys) {
+            II, II, NewImageDimIntr->Intr, IC, [&](auto &Args, auto &ArgTys) {
               Args.erase(Args.begin() + ImageDimIntr->OffsetIndex);
             });
       }
     }
   }
 
+  // Try to use D16
+  if (ST->hasD16Images()) {
+
+    const AMDGPU::MIMGBaseOpcodeInfo *BaseOpcode =
+        AMDGPU::getMIMGBaseOpcodeInfo(ImageDimIntr->BaseOpcode);
+
+    if (BaseOpcode->HasD16) {
+
+      // If the only use of image intrinsic is a fptrunc (with conversion to
+      // half) then both fptrunc and image intrinsic will be replaced with image
+      // intrinsic with D16 flag.
+      if (II.hasOneUse()) {
+        Instruction *User = II.user_back();
+
+        if (User->getOpcode() == Instruction::FPTrunc &&
+            User->getType()->getScalarType()->isHalfTy()) {
+
+          return modifyIntrinsicCall(II, *User, ImageDimIntr->Intr, IC,
+                                     [&](auto &Args, auto &ArgTys) {
+                                       // Change return type of image intrinsic.
+                                       // Set it to return type of fptrunc.
+                                       ArgTys[0] = User->getType();
+                                     });
+        }
+      }
+    }
+  }
+
   // Try to use A16 or G16
   if (!ST->hasA16() && !ST->hasG16())
     return None;
@@ -263,7 +300,7 @@ simplifyAMDGCNImageIntrinsic(const GCNSubtarget *ST,
                                : Type::getInt16Ty(II.getContext());
 
   return modifyIntrinsicCall(
-      II, II.getIntrinsicID(), IC, [&](auto &Args, auto &ArgTys) {
+      II, II, II.getIntrinsicID(), IC, [&](auto &Args, auto &ArgTys) {
         ArgTys[ImageDimIntr->GradientTyArg] = CoordType;
         if (!OnlyDerivatives) {
           ArgTys[ImageDimIntr->CoordTyArg] = CoordType;