AM3358: PRU CGT and packed attribute

Andrew Marsh

Part Number: AM3358

I have a header file containing a structure definition used by the PRU and shared with ARM. The PRU does not have alignment constraints. As a result I would expect the compiler to generate a packed struct by default. The ARM core however does have alignment constraints. So I give ARM a hint by adding __attribute__((packed)) to my struct definition.

Doing this results in the PRU pulling data byte by byte to reassemble individual values. I understand why this can happen and on certain architectures it absolutely makes sense. However given the PRU could care less about alignment shouldn't the compiler just ignore the directive?

I feel like a shared header file is a fairly common scenario. If someone isn't paying attention they are going to take a pretty significant performance hit on memory access. Granted I can #ifdef __KERNEL__ my way out of this issue quite easily. I am just curious if this is really the desired compiler behavior when specifying __attribute__((packed))?

over 7 years ago

0 Nick Saulnier over 7 years ago

TI__Guru** 104860 points

Hello Andrew,

I will check with the compiler team. What is being generated when the PRU is pulling data byte by byte?

Regards,
Nick

0 Nick Saulnier over 7 years ago in reply to Nick Saulnier

TI__Guru** 104860 points

Hello Andrew,

Let me know what the output looks like when the PRU is pulling data byte by byte, and I'll pass it on to the compiler team.

Regards,
Nick

0 Andrew Marsh over 7 years ago in reply to Nick Saulnier

Intellectual 765 points

Sorry that it took so long for me to respond. I have been out of the office and attempting to catch back up. Here is a simple example that demonstrates the behavior. Define some arbitrary data structures.

struct __attribute__ ((__packed__)) packed_struct {
	uint8_t a8;
	uint16_t b16;
	uint32_t c32;
};

struct unpacked_struct {
	uint8_t a8;
	uint16_t b16;
	uint32_t c32;
};

typedef struct {
	volatile struct packed_struct packed;
	volatile struct unpacked_struct unpacked;
} packing_test;

volatile __far packing_test test __attribute__((cregister("PRU_ARM_SMEM", near), peripheral));

do the following in your test program

	test.packed.c32 = test.unpacked.c32;
	test.unpacked.c32 = test.packed.c32;
	test.packed.b16 = test.unpacked.b16;
	test.unpacked.b16 = test.packed.b16;

compiler generates the following:

	.dwpsn	file "test.c",line 378,column 2,is_stmt,isa 0
        LDI32     r0, ||test||+3        ; [ALU_PRU] |378| test
        LBCO      &r1, __PRU_CREG_PRU_ARM_SMEM, $CSBREL(||test||+10), 4 ; [ALU_PRU] |378| test
        LSR       r14, r1, 0x08         ; [ALU_PRU] |378| 
        SBBO      &r1.b0, r0, 0, 1      ; [ALU_PRU] |378| 
        SBBO      &r14.b0, r0, 1, 1     ; [ALU_PRU] |378| 
        LSR       r14, r1, 0x10         ; [ALU_PRU] |378| 
        LSR       r1, r1, 0x18          ; [ALU_PRU] |378| 
        SBBO      &r14.b0, r0, 2, 1     ; [ALU_PRU] |378| 
        SBBO      &r1.b0, r0, 3, 1      ; [ALU_PRU] |378| 
	.dwpsn	file "test.c",line 379,column 2,is_stmt,isa 0
        LBBO      &r1.b0, r0, 1, 1      ; [ALU_PRU] |379| 
        LSL       r1, r1.b0, 0x08       ; [ALU_PRU] |379| 
        LBBO      &r14.b0, r0, 0, 1     ; [ALU_PRU] |379| 
        OR        r14, r14.b0, r1       ; [ALU_PRU] |379| 
        LBBO      &r1.b0, r0, 2, 1      ; [ALU_PRU] |379| 
        LSL       r1, r1.b0, 0x10       ; [ALU_PRU] |379| 
        OR        r1, r14, r1           ; [ALU_PRU] |379| 
        LBBO      &r0.b0, r0, 3, 1      ; [ALU_PRU] |379| 
        LSL       r0, r0.b0, 0x18       ; [ALU_PRU] |379| 
        OR        r0, r1, r0            ; [ALU_PRU] |379| 
        SBCO      &r0, __PRU_CREG_PRU_ARM_SMEM, $CSBREL(||test||+10), 4 ; [ALU_PRU] |379| test
	.dwpsn	file "test.c",line 380,column 2,is_stmt,isa 0
        LDI32     r0, ||test||+1        ; [ALU_PRU] |380| test
        LBCO      &r14.w0, __PRU_CREG_PRU_ARM_SMEM, $CSBREL(||test||+8), 2 ; [ALU_PRU] |380| test
        MOV       r1, r14.w0            ; [ALU_PRU] |380| 
        SBBO      &r1.b0, r0, 0, 1      ; [ALU_PRU] |380| 
        LSR       r1, r14.w0, 0x08      ; [ALU_PRU] |380| 
        SBBO      &r1.b0, r0, 1, 1      ; [ALU_PRU] |380| 
	.dwpsn	file "test.c",line 381,column 2,is_stmt,isa 0
        LBBO      &r1.b0, r0, 1, 1      ; [ALU_PRU] |381| 
        LSL       r1, r1.b0, 0x08       ; [ALU_PRU] |381| 
        LBBO      &r0.b0, r0, 0, 1      ; [ALU_PRU] |381| 
        OR        r0, r0.b0, r1         ; [ALU_PRU] |381| 
        SBCO      &r0.w0, __PRU_CREG_PRU_ARM_SMEM, $CSBREL(||test||+8), 2 ; [ALU_PRU] |381| test

if you drop the packed attribute from the packed_struct definition you get the following

	.dwpsn	file "test.c",line 378,column 2,is_stmt,isa 0
        LBCO      &r0, __PRU_CREG_PRU_ARM_SMEM, $CSBREL(||test||+10), 4 ; [ALU_PRU] |378| test
        SBCO      &r0, __PRU_CREG_PRU_ARM_SMEM, $CSBREL(||test||+3), 4 ; [ALU_PRU] |378| test
	.dwpsn	file "test.c",line 379,column 2,is_stmt,isa 0
        LBCO      &r0, __PRU_CREG_PRU_ARM_SMEM, $CSBREL(||test||+3), 4 ; [ALU_PRU] |379| test
        SBCO      &r0, __PRU_CREG_PRU_ARM_SMEM, $CSBREL(||test||+10), 4 ; [ALU_PRU] |379| test
	.dwpsn	file "test.c",line 380,column 2,is_stmt,isa 0
        LBCO      &r0.w0, __PRU_CREG_PRU_ARM_SMEM, $CSBREL(||test||+8), 2 ; [ALU_PRU] |380| test
        SBCO      &r0.w0, __PRU_CREG_PRU_ARM_SMEM, $CSBREL(||test||+1), 2 ; [ALU_PRU] |380| test
	.dwpsn	file "test.c",line 381,column 2,is_stmt,isa 0
        LBCO      &r0.w0, __PRU_CREG_PRU_ARM_SMEM, $CSBREL(||test||+1), 2 ; [ALU_PRU] |381| test
        SBCO      &r0.w0, __PRU_CREG_PRU_ARM_SMEM, $CSBREL(||test||+8), 2 ; [ALU_PRU] |381| test

0 Andrew Marsh over 7 years ago in reply to Andrew Marsh

Intellectual 765 points

Just to clarify, the shared memory is located in PRU0 Data RAM.

0 Nick Saulnier over 7 years ago in reply to Andrew Marsh

TI__Guru** 104860 points

Hello Andrew,

Great, thank you for bringing this up. The compiler team says this is a gap in the compiler: You are right, the PRU assembly for a packed and unpacked structure should not result in that different assembly. I have submitted a bug report, so it should get addressed sometime down the road. For now, your #ifdef workaround sounds reasonable.

Regards,
Nick

Processors

Processors forum

AM3358: PRU CGT and packed attribute