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‎llvm/docs/AVRTarget.rst

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=================================
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Documentation for the AVR backend
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=================================
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.. contents::
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:local:
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Calling convention
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==================
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External links
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--------------
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* The avr-gcc .. _wiki: https://gcc.gnu.org/wiki/avr-gcc#Calling_Convention has a description of the C calling convention they use
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* The avr-libc .. documentation: http://www.nongnu.org/avr-libc/user-manual/FAQ.html#faq_reg_usage has a small summary of the C calling convention
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Function return values
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----------------------
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The avr-gcc documentation states that
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Return values with a size of 1 byte up to and including a size of 8 bytes will be returned in registers. Return values whose size is outside that range will be returned in memory.
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-- avr-gcc wiki, October 2018
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This does not strictly seem to be the case. Scalar return values like i8, i16, and i64 do indeed seem to follow this rule, according to the output of avr-gcc. Integer return values of up to 64-bits (8 bytes) are always returned directly in registers. However, when a struct is being returned, only structs of 4 bytes or less are returned in registers. If the struct is of 5 bytes or larger, avr-gcc will always pass it on the stack.
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This means that different return types have different size limits for fitting into registers or the stack.

‎llvm/lib/Target/AVR/AVRISelLowering.cpp

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Original file line numberDiff line numberDiff line change
@@ -1305,6 +1305,35 @@ SDValue AVRTargetLowering::LowerCall(TargetLowering::CallLoweringInfo &CLI,
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InVals);
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}
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/// Reverse splitted return values to get the "big endian" format required
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/// to agree with the calling convention ABI.
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static void ReverseArgumentsToBigEndian(MachineFunction &MF,
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SmallVector<CCValAssign, 16> &RVLocs) {
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if (RVLocs.size() > 1) {
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// Some hackery because SelectionDAGBuilder does not split
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// up arguments properly
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Type *retType = MF.getFunction().getReturnType();
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if (retType->isStructTy()) {
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if (retType->getNumContainedTypes() > 1 &&
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retType->getNumContainedTypes() > RVLocs.size()) {
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for (unsigned i = 0, pos = 0;
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i < retType->getNumContainedTypes(); ++i) {
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Type *field = retType->getContainedType(i);
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if(field->isIntegerTy() && field->getIntegerBitWidth() > 16) {
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int Size = field->getIntegerBitWidth() / 16;
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std::reverse(RVLocs.begin()+ pos, RVLocs.end() + pos + Size);
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pos += Size;
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} else {
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pos++;
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}
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}
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}
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} else {
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std::reverse(RVLocs.begin(), RVLocs.end());
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}
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}
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}
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/// Lower the result values of a call into the
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/// appropriate copies out of appropriate physical registers.
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///
@@ -1322,12 +1351,6 @@ SDValue AVRTargetLowering::LowerCallResult(
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auto CCFunction = CCAssignFnForReturn(CallConv);
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CCInfo.AnalyzeCallResult(Ins, CCFunction);
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if (CallConv != CallingConv::AVR_BUILTIN && RVLocs.size() > 1) {
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// Reverse splitted return values to get the "big endian" format required
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// to agree with the calling convention ABI.
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std::reverse(RVLocs.begin(), RVLocs.end());
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}
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// Copy all of the result registers out of their specified physreg.
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for (CCValAssign const &RVLoc : RVLocs) {
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Chain = DAG.getCopyFromReg(Chain, dl, RVLoc.getLocReg(), RVLoc.getValVT(),
@@ -1390,9 +1413,7 @@ AVRTargetLowering::LowerReturn(SDValue Chain, CallingConv::ID CallConv,
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// Reverse splitted return values to get the "big endian" format required
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// to agree with the calling convention ABI.
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if (e > 1) {
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std::reverse(RVLocs.begin(), RVLocs.end());
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}
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ReverseArgumentsToBigEndian(MF, RVLocs);
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SDValue Flag;
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SmallVector<SDValue, 4> RetOps(1, Chain);

‎llvm/test/CodeGen/AVR/calling-conv/c/return.ll

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@@ -34,3 +34,117 @@ define i64 @ret_i64() {
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; CHECK-NEXT: mov r25, r19
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ret i64 18374966859414961920
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}
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; CHECK-LABEL: ret_struct1
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define { i8, i32 } @ret_struct1() {
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start:
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; CHECK: ldi r22, 221
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; CHECK: ldi r23, 204
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; CHECK: ldi r20, 187
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; CHECK: ldi r21, 170
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; CHECK: ldi r24, 111
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ret { i8, i32 } { i8 111, i32 2864434397 } ; { 0x6f, 0xaabbccdd }
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}
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; CHECK-LABEL: ret_struct2
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define { i8, i8, i8, i8, i8 } @ret_struct2() {
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start:
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ret { i8, i8, i8, i8, i8 } { i8 111, i8 221, i8 204, i8 187, i8 170 } ; { 0x6f, 0xaabbccdd }
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}
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; CHECK-LABEL: ret_struct3
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define { i32, i32 } @ret_struct3() {
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start:
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ret { i32, i32 } zeroinitializer
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}
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; CHECK-LABEL: ret_struct4
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define { i8, i64 } @ret_struct4() {
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start:
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ret { i8, i64 } zeroinitializer
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}
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; CHECK-LABEL: ret_struct5
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define { i8, i32, i16 } @ret_struct5() {
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start:
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ret { i8, i32, i16 } zeroinitializer
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}
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; CHECK-LABEL: ret_struct6
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define { i8, i32, i32 } @ret_struct6() {
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start:
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ret { i8, i32, i32 } zeroinitializer
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}
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; Structs of size 1 should be returned in registers.
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;
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; This matches avr-gcc behaviour.
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;
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; CHECK-LABEL: ret_struct_size1
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define { i8 } @ret_struct_size1() {
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; CHECK: ldi r24, 123
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ret { i8 } { i8 123 }
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}
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; Structs of size 2 are currently returned on the stack.
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;
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; BUG(PR39251): avr-gcc returns all structs of four bytes or less in registers.
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;
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; CHECK-LABEL: ret_struct_size2
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define { i8, i8 } @ret_struct_size2() {
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ret { i8, i8 } { i8 123, i8 234 }
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}
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; Structs of size 3 are currently returned on the stack.
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;
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; BUG(PR39251): avr-gcc returns all structs of four bytes or less in registers.
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;
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; CHECK-LABEL: ret_struct_size3
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define { i8, i8, i8 } @ret_struct_size3() {
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ret { i8, i8, i8 } { i8 123, i8 234, i8 255 }
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}
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; Structs of size 4 are currently returned on the stack.
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;
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; BUG(PR39251): avr-gcc returns all structs of four bytes or less in registers.
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;
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; CHECK-LABEL: ret_struct_size4
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define { i8, i8, i8, i8 } @ret_struct_size4() {
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ret { i8, i8, i8, i8 } { i8 123, i8 234, i8 255, i8 11 }
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}
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; Structs of size 5 should be returned on the stack.
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;
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; This matches avr-gcc behaviour.
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;
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; CHECK-LABEL: ret_struct_size5
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define { i8, i8, i8, i8, i8 } @ret_struct_size5() {
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ret { i8, i8, i8, i8, i8 } { i8 123, i8 234, i8 255, i8 11, i8 22 }
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}
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; Structs of size 6 should be returned on the stack.
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;
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; This matches avr-gcc behaviour.
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;
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; CHECK-LABEL: ret_struct_size6
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define { i8, i8, i8, i8, i8, i8 } @ret_struct_size6() {
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ret { i8, i8, i8, i8, i8, i8 } { i8 123, i8 234, i8 255, i8 11, i8 22, i8 33 }
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}
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; Structs of size 7 should be returned on the stack.
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;
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; This matches avr-gcc behaviour.
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;
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; CHECK-LABEL: ret_struct_size7
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define { i8, i8, i8, i8, i8, i8, i8 } @ret_struct_size7() {
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ret { i8, i8, i8, i8, i8, i8, i8 } { i8 123, i8 234, i8 255, i8 11, i8 22, i8 33, i8 44 }
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}
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; Structs of size 8 should be returned on the stack.
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;
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; This matches avr-gcc behaviour.
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;
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; CHECK-LABEL: ret_struct_size8
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define { i8, i8, i8, i8, i8, i8, i8, i8 } @ret_struct_size8() {
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ret { i8, i8, i8, i8, i8, i8, i8, i8 } { i8 123, i8 234, i8 255, i8 11, i8 22, i8 33, i8 44, i8 55 }
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}

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