Counting register number in PTX

I am sorry to open a new thread, but I didn’t find anything regarding this.

How can I calculate exactly the number or register in PTX?


Here I get:

ptxas info    : Compiling entry function '_Z12Karatsuba1x1Pj' for 'sm_10'

1>  ptxas info    : Used 10 registers, 8+16 bytes smem, 12 bytes cmem[0], 8 bytes cmem[1], 8 bytes cmem[14]

Why 10 registers? I should have 4 (2 inputs + 2 outputs) plus 2 (double register d1)

Where am I wrong?

Which toolkit version are you using? I’ve just run the kernel through 3.2 and got 7 registers:

[font=“Courier New”]Compiling entry function ‘_Z12Karatsuba1x1Pj’ for ‘sm_10’

ptxas info : Used 7 registers, 4+16 bytes smem, 4 bytes cmem[1]


You can use [font=“Courier New”]decuda[/font]

// Disassembling _Z12Karatsuba1x1Pj

000000: 41002c05 00000007 mul24.lo.u32.u16.u16 $r1, s[0x000c], 0x0040

000008: a0000001 04000780 cvt.rn.u32.u16 $r0, $r0.lo

000010: 20000001 04004780 add.u32 $r0, $r0, $r1

000018: 30030001 c4100780 shl.u32 $r0, $r0, 0x00000003

000020: 2000c80d 04200780 add.u32 $r3, s[0x0010], $r0

000028: 20048609 00000003 add.b32 $r2, $r3, 0x00000004

000030: d00e0601 80c00780 mov.u32 $r0, g[$r3]

000038: d00e0405 80c00780 mov.u32 $r1, g[$r2]

000040: 40030005 00000780 mul24.lo.u32.u16.u16 $r1, $r0.lo, $r1.hi

000048: 60020211 000147c0 mad24.lo.u32.u16.u16.u32 $p0|$r4, $r0.hi, $r1.lo, $r5

000050: 30100809 c4100780 shl.u32 $r2, $r4, 0x00000010

000058: 30100811 e4100780 shr.u32 $r4, $r4, 0x00000010

000060: 6002010d 000187d8 mad24.lo.u32.u16.u16.u32 $p1|$o67, $r0.lo, $r1.lo, $r6

000068: 60020215 00014780 mad24.lo.u32.u16.u16.u32 $r5, $r0.hi, $r1.lo, $r5

000070: 21000801 04400880 @$ add.u32 $r0, $r4, c1[0x0000]

000078: 30100a15 c4100780 shl.u32 $r5, $r5, 0x00000010

000080: 60030201 0c011780 mad24c1.lo.u32.u16.u16.u32 $r0, -$r0.hi, $r1.hi, -$r4

000088: 60020001 00014780 mad24.lo.u32.u16.u16.u32 $r0, $r0.lo, $r1.lo, $r5

000090: d00e0601 a0c00780 mov.u32 g[$r3], $r0

000098: d00e0401 a0c00781 mov.end.u32 g[$r2], $r0

// segment: const (0:0000)

0000: 00010000

or [font=“Courier New”]cuobjdump[/font] to find out yourself what the registers are used for:

code for sm_10

		Function : _Z12Karatsuba1x1Pj

	/*0000*/     /*0x41002c0500000007*/ 	IMUL32I.U16.U16 R1, g [0x6].U16, 0x40;

	/*0008*/     /*0xa000000104000780*/ 	I2I.U32.U16 R0, R0L;

	/*0010*/     /*0x2000000104004780*/ 	IADD R0, R0, R1;

	/*0018*/     /*0x30030001c4100780*/ 	SHL R0, R0, 0x3;

	/*0020*/     /*0x2000c80d04200780*/ 	IADD R3, g [0x4], R0;

	/*0028*/     /*0x2004860900000003*/ 	IADD32I R2, R3, 0x4;

	/*0030*/     /*0xd00e060180c00780*/ 	GLD.U32 R0, global14 [R3];

	/*0038*/     /*0xd00e040580c00780*/ 	GLD.U32 R1, global14 [R2];

	/*0040*/     /*0x4003001500000780*/ 	IMUL.U16.U16 R5, R0L, R1H;

	/*0048*/     /*0x60020211000147c0*/ 	IMAD.U16.C0 R4, R0H, R1L, R5;

	/*0050*/     /*0x30100819c4100780*/ 	SHL R6, R4, 0x10;

	/*0058*/     /*0x30100811e4100780*/ 	SHR R4, R4, 0x10;

	/*0060*/     /*0x600201fd000187d8*/ 	IMAD.U16.C1 o [0x7f], R0L, R1L, R6;

	/*0068*/     /*0x6002021500014780*/ 	IMAD.U16 R5, R0H, R1L, R5;

	/*0070*/     /*0x2100081104400880*/ 	IADD R4 (C0.CARRY), R4, c [0x1] [0x0];

	/*0078*/     /*0x30100a15c4100780*/ 	SHL R5, R5, 0x10;

	/*0080*/     /*0x600302110c011780*/ 	IMAD.U16 R4 (C3), R0H, R1H, -R4;

	/*0088*/     /*0x6002000100014780*/ 	IMAD.U16 R0, R0L, R1L, R5;

	/*0090*/     /*0xd00e0611a0c00780*/ 	GST.U32 global14 [R3], R4;

	/*0098*/     /*0xd00e0401a0c00781*/ 	GST.U32 global14 [R2], R0;

3.2, 4.0 doesnt matter, I always get 10…


Another problem is that I declare in my kernel

__shared__ unsigned int x[8704];

But after compiling I get this error:

1>  ptxas info    : Compiling entry function '_Z12Karatsuba2x2Pj' for 'sm_13'

1>  ptxas info    : Used 17 registers, 34824+16 bytes smem, 12 bytes cmem[0], 40 bytes cmem[1], 8 bytes cmem[14]

1>CUDACOMPILE : ptxas error : Entry function '_Z12Karatsuba2x2Pj' uses too much shared data (0x8808 bytes + 0x10 bytes system, 0x4000 max)

I declared 8704 Bytes length (in decimal) but PTX try to compile interpreting it like an hexadecimal number! (8704 is ca. 8808, that is exactly 34824…)

Why? Bug?

The size of an unsigned integer is 4 bytes, so when you declare the array x[8704], that requires 4 * 8704 = 34816 bytes of shared memory. The relation to hexadecimal is a numerical coincidence in this integer range.

A (belated) answer to the original question is that one cannot determine the register count for any kernel from simply looking at PTX code, since register allocation occurs as part of the translation from PTX to machine code. Adding -Xptxas -v to the nvcc commandline enables the printing of register usage statistics, and one can use cuobjdump to examine how registers are allocated inside the machine code (as tera already pointed out).