[clang][bytecode] Slightly optimize integral casts of literals (#138879)

We often see initializers like

unsigned a = 10;

which take an integer literal and immediately cast it to another type.
Recognize this pattern and omit the cast, simply emitting the value as a
different type directly.

This reduces the instruction count by up to 0.13%:
http://llvm-compile-time-tracker.com/compare.php?from=303436c6d16518b35288d63a859506ffcc1681e4&to=648f5202f906d1606390b2d1081e4502dc74acc2&stat=instructions:u

Author: tbaederr

clang/lib/AST/ByteCode/Compiler.cpp

@@ -238,8 +238,8 @@ bool Compiler<Emitter>::VisitCastExpr(const CastExpr *CE) {
   case CK_DerivedToBaseMemberPointer: {
     assert(classifyPrim(CE->getType()) == PT_MemberPtr);
     assert(classifyPrim(SubExpr->getType()) == PT_MemberPtr);
-    const auto *FromMP = SubExpr->getType()->getAs<MemberPointerType>();
-    const auto *ToMP = CE->getType()->getAs<MemberPointerType>();
+    const auto *FromMP = SubExpr->getType()->castAs<MemberPointerType>();
+    const auto *ToMP = CE->getType()->castAs<MemberPointerType>();
 
     unsigned DerivedOffset =
         Ctx.collectBaseOffset(ToMP->getMostRecentCXXRecordDecl(),
@@ -254,8 +254,8 @@ bool Compiler<Emitter>::VisitCastExpr(const CastExpr *CE) {
   case CK_BaseToDerivedMemberPointer: {
     assert(classifyPrim(CE) == PT_MemberPtr);
     assert(classifyPrim(SubExpr) == PT_MemberPtr);
-    const auto *FromMP = SubExpr->getType()->getAs<MemberPointerType>();
-    const auto *ToMP = CE->getType()->getAs<MemberPointerType>();
+    const auto *FromMP = SubExpr->getType()->castAs<MemberPointerType>();
+    const auto *ToMP = CE->getType()->castAs<MemberPointerType>();
 
     unsigned DerivedOffset =
         Ctx.collectBaseOffset(FromMP->getMostRecentCXXRecordDecl(),
@@ -320,37 +320,38 @@ bool Compiler<Emitter>::VisitCastExpr(const CastExpr *CE) {
   }
 
   case CK_IntegralToFloating: {
-    std::optional<PrimType> FromT = classify(SubExpr->getType());
-    if (!FromT)
+    if (!CE->getType()->isRealFloatingType())
       return false;
-
     if (!this->visit(SubExpr))
       return false;
-
     const auto *TargetSemantics = &Ctx.getFloatSemantics(CE->getType());
-    return this->emitCastIntegralFloating(*FromT, TargetSemantics,
-                                          getFPOptions(CE), CE);
+    return this->emitCastIntegralFloating(
+        classifyPrim(SubExpr), TargetSemantics, getFPOptions(CE), CE);
   }
 
-  case CK_FloatingToBoolean:
-  case CK_FloatingToIntegral: {
-
-    std::optional<PrimType> ToT = classify(CE->getType());
-
-    if (!ToT)
+  case CK_FloatingToBoolean: {
+    if (!SubExpr->getType()->isRealFloatingType() ||
+        !CE->getType()->isBooleanType())
       return false;
-
+    if (const auto *FL = dyn_cast<FloatingLiteral>(SubExpr))
+      return this->emitConstBool(FL->getValue().isNonZero(), CE);
     if (!this->visit(SubExpr))
       return false;
+    return this->emitCastFloatingIntegralBool(getFPOptions(CE), CE);
+  }
 
+  case CK_FloatingToIntegral: {
+    if (!this->visit(SubExpr))
+      return false;
+    PrimType ToT = classifyPrim(CE);
     if (ToT == PT_IntAP)
       return this->emitCastFloatingIntegralAP(Ctx.getBitWidth(CE->getType()),
                                               getFPOptions(CE), CE);
     if (ToT == PT_IntAPS)
       return this->emitCastFloatingIntegralAPS(Ctx.getBitWidth(CE->getType()),
                                                getFPOptions(CE), CE);
 
-    return this->emitCastFloatingIntegral(*ToT, getFPOptions(CE), CE);
+    return this->emitCastFloatingIntegral(ToT, getFPOptions(CE), CE);
   }
 
   case CK_NullToPointer:
@@ -395,9 +396,7 @@ bool Compiler<Emitter>::VisitCastExpr(const CastExpr *CE) {
   case CK_ArrayToPointerDecay: {
     if (!this->visit(SubExpr))
       return false;
-    if (!this->emitArrayDecay(CE))
-      return false;
-    return true;
+    return this->emitArrayDecay(CE);
   }
 
   case CK_IntegralToPointer: {
@@ -480,47 +479,63 @@ bool Compiler<Emitter>::VisitCastExpr(const CastExpr *CE) {
     return this->emitBuiltinBitCast(CE);
 
   case CK_IntegralToBoolean:
-  case CK_FixedPointToBoolean:
+  case CK_FixedPointToBoolean: {
+    // HLSL uses this to cast to one-element vectors.
+    std::optional<PrimType> FromT = classify(SubExpr->getType());
+    if (!FromT)
+      return false;
+
+    if (const auto *IL = dyn_cast<IntegerLiteral>(SubExpr))
+      return this->emitConst(IL->getValue(), CE);
+    if (!this->visit(SubExpr))
+      return false;
+    return this->emitCast(*FromT, classifyPrim(CE), CE);
+  }
+
   case CK_BooleanToSignedIntegral:
   case CK_IntegralCast: {
     std::optional<PrimType> FromT = classify(SubExpr->getType());
     std::optional<PrimType> ToT = classify(CE->getType());
-
     if (!FromT || !ToT)
       return false;
 
-    if (!this->visit(SubExpr))
-      return false;
+    // Try to emit a casted known constant value directly.
+    if (const auto *IL = dyn_cast<IntegerLiteral>(SubExpr)) {
+      if (ToT != PT_IntAP && ToT != PT_IntAPS && FromT != PT_IntAP &&
+          FromT != PT_IntAPS && !CE->getType()->isEnumeralType())
+        return this->emitConst(IL->getValue(), CE);
+      if (!this->emitConst(IL->getValue(), SubExpr))
+        return false;
+    } else {
+      if (!this->visit(SubExpr))
+        return false;
+    }
 
     // Possibly diagnose casts to enum types if the target type does not
     // have a fixed size.
     if (Ctx.getLangOpts().CPlusPlus && CE->getType()->isEnumeralType()) {
-      if (const auto *ET = CE->getType().getCanonicalType()->getAs<EnumType>();
-          ET && !ET->getDecl()->isFixed()) {
+      if (const auto *ET = CE->getType().getCanonicalType()->castAs<EnumType>();
+          !ET->getDecl()->isFixed()) {
         if (!this->emitCheckEnumValue(*FromT, ET->getDecl(), CE))
           return false;
       }
     }
 
-    auto maybeNegate = [&]() -> bool {
-      if (CE->getCastKind() == CK_BooleanToSignedIntegral)
-        return this->emitNeg(*ToT, CE);
-      return true;
-    };
-
-    if (ToT == PT_IntAP)
-      return this->emitCastAP(*FromT, Ctx.getBitWidth(CE->getType()), CE) &&
-             maybeNegate();
-    if (ToT == PT_IntAPS)
-      return this->emitCastAPS(*FromT, Ctx.getBitWidth(CE->getType()), CE) &&
-             maybeNegate();
-
-    if (FromT == ToT)
-      return true;
-    if (!this->emitCast(*FromT, *ToT, CE))
-      return false;
-
-    return maybeNegate();
+    if (ToT == PT_IntAP) {
+      if (!this->emitCastAP(*FromT, Ctx.getBitWidth(CE->getType()), CE))
+        return false;
+    } else if (ToT == PT_IntAPS) {
+      if (!this->emitCastAPS(*FromT, Ctx.getBitWidth(CE->getType()), CE))
+        return false;
+    } else {
+      if (FromT == ToT)
+        return true;
+      if (!this->emitCast(*FromT, *ToT, CE))
+        return false;
+    }
+    if (CE->getCastKind() == CK_BooleanToSignedIntegral)
+      return this->emitNeg(*ToT, CE);
+    return true;
   }
 
   case CK_PointerToBoolean: