PTX ISA special-registers

PTX ISA Special Registers

综述

PTX includes a number of predefined, read-only variables, which are visible as special registers and accessed through mov or cvt instructions.

  • %tid      

    • CTA 内的线程号 
  • %ntid      

    • CTA 内的线程数 
  • %laneid     

    • warp内的线程号
  • %warpid     

    • CTA 内的warp号
  • %nwarpid    

    • CTA 内的warp数 
  • %ctaid   

    • CTA 号 
  • %nctaid     

    • CTA 数 
  • %smid      

    • SM 号
  • %nsmid     

    • SM数
  • %gridid     

    • 网格号 包含 多个CTA
  • %lanemask_eq, %lanemask_le, %lanemask_lt, %lanemask_ge, %lanemask_gt

    • 从fermi架构开始提出的线程掩码寄存器,大小32位,我表示每个warp中 active 的线程数。
    • 不大常见
  • %clock, %clock_hi, %clock64

    • 无符号预定义只读的 计数寄存器
    • %clock      32位
    • %clock_hi   高32位
    • %clock64     64位
  • %pm0, ..., %pm7

    • performance monitoring counters
    • 性能监视计数器,没用过。。
  • %pm0_64, ..., %pm7_64

    • 64 bit performance monitoring counters
    • 性能监视计数器,没用过。。
  • %envreg0, ..., %envreg31

    • Driver-defined read-only registers.
    • 驱动器只读寄存器
    • 用来获取PTX程序运行环境,ARCH?
  • %globaltimer, %globaltimer_lo, %globaltimer_hi

    • A predefined, 64-bit global nanosecond timer.
    • 全局纳秒寄存器
    • 64 bit low 32 bit high 32 bit
  • %total_smem_size     

    • 每个CTA使用shared memory 的大小 举个例子 fermi 16K/32K
  • %dynamic_smem_size  

    • 这个是kernel 函数启动时动态分批的总大小

看完综述应该大体上知道这些都是干嘛的了,如果还想详细了解,再往下看。

9.1. Special Registers: %tid

%tid
​ Thread identifier within a CTA.

Syntax (predefined)

.sreg .v4 .u32 %tid; // thread id vector
.sreg .u32 %tid.x, %tid.y, %tid.z; // thread id components

Description

A predefined, read-only, per-thread special register initialized with the thread identifier within the CTA. The %tid special register contains a 1D, 2D, or 3D vector to match the CTA shape; the %tid value in unused dimensions is 0. The fourth element is unused and always returns zero. The number of threads in each dimension are specified by the predefined special register %ntid.

Every thread in the CTA has a unique %tid.

%tid component values range from 0 through %ntid-1 in each CTA dimension.

%tid.y == %tid.z == 0 in 1D CTAs. %tid.z == 0 in 2D CTAs.

It is guaranteed that:

0 <= %tid.x < %ntid.x
0 <= %tid.y < %ntid.y
0 <= %tid.z < %ntid.z

PTX ISA Notes

Introduced in PTX ISA version 1.0 with type .v4.u16.

Redefined as type .v4.u32 in PTX ISA version 2.0. For compatibility with legacy PTX code, 16-bit mov and cvt instructions may be used to read the lower 16-bits of each component of %tid.

Target ISA Notes

Supported on all target architectures.

Examples

9.2. Special Registers: %ntid

%ntid
Number of thread IDs per CTA.

Syntax (predefined)

.sreg .v4 .u32 %ntid; // CTA shape vector
.sreg .u32 %ntid.x, %ntid.y, %ntid.z; // CTA dimensions

Description

A predefined, read-only special register initialized with the number of thread ids in each CTA dimension. The %ntid special register contains a 3D CTA shape vector that holds the CTA dimensions. CTA dimensions are non-zero; the fourth element is unused and always returns zero. The total number of threads in a CTA is (%ntid.x * %ntid.y * %ntid.z).

%ntid.y == %ntid.z == 1 in 1D CTAs.
%ntid.z ==1 in 2D CTAs.
Maximum values of %ntid.{x,y,z} are as follows:

.target architecture %ntid.x %ntid.y %ntid.z
sm_1x 512 512 64
sm20, sm3x, sm5x, sm6x 1024 1024 64

PTX ISA Notes

Introduced in PTX ISA version 1.0 with type .v4.u16.

Redefined as type .v4.u32 in PTX ISA version 2.0. For compatibility with legacy PTX code, 16-bit mov and cvt instructions may be used to read the lower 16-bits of each component of %ntid.

Target ISA Notes

Supported on all target architectures.

Examples

9.3. Special Registers: %laneid

%laneid
Lane Identifier.

Syntax (predefined)

.sreg .u32 %laneid;

Description

A predefined, read-only special register that returns the thread's lane within the warp. The lane identifier ranges from zero to WARP_SZ-1.

PTX ISA Notes

Introduced in PTX ISA version 1.3.

Target ISA Notes

Supported on all target architectures.

Examples

9.4. Special Registers: %warpid

%warpid
Warp identifier.

Syntax (predefined)

.sreg .u32 %warpid;
Description
A predefined, read-only special register that returns the thread's warp identifier. The warp identifier provides a unique warp number within a CTA but not across CTAs within a grid. The warp identifier will be the same for all threads within a single warp.

Note that %warpid is volatile and returns the location of a thread at the moment when read, but its value may change during execution, e.g., due to rescheduling of threads following preemption.For this reason, %ctaid and %tid should be used to compute a virtual warp index if such a value is needed in kernel code; %warpid is intended mainly to enable profiling and diagnostic code to sample and log information such as work place mapping and load distribution.

PTX ISA Notes

Introduced in PTX ISA version 1.3.

Target ISA Notes

Supported on all target architectures.

Examples

9.5. Special Registers: %nwarpid

%nwarpid
Number of warp identifiers.

Syntax (predefined)

.sreg .u32 %nwarpid;

Description

A predefined, read-only special register that returns the maximum number of warp identifiers.

PTX ISA Notes

Introduced in PTX ISA version 2.0.

Target ISA Notes

%nwarpid requires sm_20 or higher.

Examples

mov.u32 %r, %nwarpid;

9.6. Special Registers: %ctaid

%ctaid
CTA identifier within a grid.

Syntax (predefined)

.sreg .v4 .u32 %ctaid; // CTA id vector
.sreg .u32 %ctaid.x, %ctaid.y, %ctaid.z; // CTA id components

Description

A predefined, read-only special register initialized with the CTA identifier within the CTA grid. The %ctaid special register contains a 1D, 2D, or 3D vector, depending on the shape and rank of the CTA grid. The fourth element is unused and always returns zero.

It is guaranteed that:

0 <= %ctaid.x < %nctaid.x
0 <= %ctaid.y < %nctaid.y
0 <= %ctaid.z < %nctaid.z

PTX ISA Notes

Introduced in PTX ISA version 1.0 with type .v4.u16.

Redefined as type .v4.u32 in PTX ISA version 2.0. For compatibility with legacy PTX code, 16-bit mov and cvt instructions may be used to read the lower 16-bits of each component of %ctaid.

Target ISA Notes

Supported on all target architectures.

Examples

9.7. Special Registers: %nctaid

%nctaid
Number of CTA ids per grid.

Syntax (predefined)

.sreg .v4 .u32 %nctaid // Grid shape vector
.sreg .u32 %nctaid.x,%nctaid.y,%nctaid.z; // Grid dimensions

Description

A predefined, read-only special register initialized with the number of CTAs in each grid dimension. The %nctaid special register contains a 3D grid shape vector, with each element having a value of at least 1. The fourth element is unused and always returns zero.

Maximum values of %nctaid.{x,y,z} are as follows:

.target architecture %nctaid.x %nctaid.y %nctaid.z
sm1x, sm20 65535 65535 65535
sm3x, sm5x, sm_6x 2^31 -1 65535 65535

PTX ISA Notes

Introduced in PTX ISA version 1.0 with type .v4.u16.

Redefined as type .v4.u32 in PTX ISA version 2.0. For compatibility with legacy PTX code, 16-bit mov and cvt instructions may be used to read the lower 16-bits of each component of %nctaid.

Target ISA Notes

Supported on all target architectures.

Examples

9.8. Special Registers: %smid

%smid
SM identifier.

Syntax (predefined)

.sreg .u32 %smid;

Description

A predefined, read-only special register that returns the processor (SM) identifier on which a particular thread is executing. The SM identifier ranges from 0 to %nsmid-1. The SM identifier numbering is not guaranteed to be contiguous.

Notes

Note that %smid is volatile and returns the location of a thread at the moment when read, but its value may change during execution, e.g. due to rescheduling of threads following preemption. %smid is intended mainly to enable profiling and diagnostic code to sample and log information such as work place mapping and load distribution.

PTX ISA Notes

Introduced in PTX ISA version 1.3.

Target ISA Notes

Supported on all target architectures.

Examples

9.9. Special Registers: %nsmid

%nsmid
Number of SM identifiers.

Syntax (predefined)

.sreg .u32 %nsmid;

Description

A predefined, read-only special register that returns the maximum number of SM identifiers. The SM identifier numbering is not guaranteed to be contiguous, so %nsmid may be larger than the physical number of SMs in the device.

PTX ISA Notes

Introduced in PTX ISA version 2.0.

Target ISA Notes

%nsmid requires sm_20 or higher.

Examples

9.10. Special Registers: %gridid

%gridid
Grid identifier.

Syntax (predefined)

.sreg .u64 %gridid;

Description

A predefined, read-only special register initialized with the per-grid temporal grid identifier. The %gridid is used by debuggers to distinguish CTAs within concurrent (small) CTA grids.

During execution, repeated launches of programs may occur, where each launch starts a grid-of-CTAs. This variable provides the temporal grid launch number for this context.

For sm1x targets, %gridid is limited to the range [0..216-1]. For sm20, %gridid is limited to the range [0..232-1]. sm_30 supports the entire 64-bit range.

PTX ISA Notes

Introduced in PTX ISA version 1.0 as type .u16.

Redefined as type .u32 in PTX ISA version 1.3.

Redefined as type .u64 in PTX ISA version 3.0.

For compatibility with legacy PTX code, 16-bit and 32-bit mov and cvt instructions may be used to read the lower 16-bits or 32-bits of each component of %gridid.

Target ISA Notes

Supported on all target architectures.

Examples

9.11. Special Registers: %lanemask_eq

%lanemask_eq
32-bit mask with bit set in position equal to the thread's lane number in the warp.

Syntax (predefined)

.sreg .u32 %lanemask_eq;

Description

A predefined, read-only special register initialized with a 32-bit mask with a bit set in the position equal to the thread's lane number in the warp.

PTX ISA Notes

Introduced in PTX ISA version 2.0.

Target ISA Notes

%lanemask_eq requires sm_20 or higher.

Examples

9.12. Special Registers: %lanemask_le

%lanemask_le
32-bit mask with bits set in positions less than or equal to the thread's lane number in the warp.

Syntax (predefined)

.sreg .u32 %lanemask_le;

Description

A predefined, read-only special register initialized with a 32-bit mask with bits set in positions less than or equal to the thread's lane number in the warp.

PTX ISA Notes

Introduced in PTX ISA version 2.0.

Target ISA Notes

%lanemask_le requires sm_20 or higher.

Examples

9.13. Special Registers: %lanemask_lt

%lanemask_lt
32-bit mask with bits set in positions less than the thread's lane number in the warp.

Syntax (predefined)

.sreg .u32 %lanemask_lt;

Description

A predefined, read-only special register initialized with a 32-bit mask with bits set in positions less than the thread's lane number in the warp.

PTX ISA Notes

Introduced in PTX ISA version 2.0.

Target ISA Notes

%lanemask_lt requires sm_20 or higher.

Examples

9.14. Special Registers: %lanemask_ge

%lanemask_ge
32-bit mask with bits set in positions greater than or equal to the thread's lane number in the warp.

Syntax (predefined)

.sreg .u32 %lanemask_ge;

Description

A predefined, read-only special register initialized with a 32-bit mask with bits set in positions greater than or equal to the thread's lane number in the warp.

PTX ISA Notes

Introduced in PTX ISA version 2.0.

Target ISA Notes

%lanemask_ge requires sm_20 or higher.

Examples

9.15. Special Registers: %lanemask_gt

%lanemask_gt
32-bit mask with bits set in positions greater than the thread's lane number in the warp.

Syntax (predefined)

.sreg .u32 %lanemask_gt;

Description

A predefined, read-only special register initialized with a 32-bit mask with bits set in positions greater than the thread's lane number in the warp.

PTX ISA Notes

Introduced in PTX ISA version 2.0.

Target ISA Notes

%lanemask_gt requires sm_20 or higher.

Examples

9.16. Special Registers: %clock, %clock_hi

%clock, %clock_hi
%clock
A predefined, read-only 32-bit unsigned cycle counter.
%clock_hi
The upper 32-bits of %clock64 special register.

Syntax (predefined)

.sreg .u32 %clock;
.sreg .u32 %clock_hi;

Description

Special register %clock and %clock_hi are unsigned 32-bit read-only cycle counters that wrap silently.

PTX ISA Notes

%clock introduced in PTX ISA version 1.0.

%clock_hi introduced in PTX ISA version 5.0.

Target ISA Notes

%clock supported on all target architectures.

%clock_hi requires sm_20 or higher.

Examples

9.17. Special Registers: %clock64

%clock64
A predefined, read-only 64-bit unsigned cycle counter.

Syntax (predefined)

.sreg .u64 %clock64;

Description

Special register %clock64 is an unsigned 64-bit read-only cycle counter that wraps silently.

Notes

The lower 32-bits of %clock64 are identical to %clock.

The upper 32-bits of %clock64 are identical to %clock_hi.

PTX ISA Notes

Introduced in PTX ISA version 2.0.

Target ISA Notes

%clock64 requires sm_20 or higher.

Examples

9.18. Special Registers: %pm0..%pm7

%pm0..%pm7
Performance monitoring counters.

Syntax (predefined)

.sreg .u32 %pm<8>;

Description

Special registers %pm0..%pm7 are unsigned 32-bit read-only performance monitor counters. Their behavior is currently undefined.

PTX ISA Notes

%pm0..%pm3 introduced in PTX ISA version 1.3.

%pm4..%pm7 introduced in PTX ISA version 3.0.

Target ISA Notes

%pm0..%pm3 supported on all target architectures.

%pm4..%pm7 require sm_20 or higher.

Examples

9.19. Special Registers: %pm064..%pm764

%pm0_64..%pm7_64
64 bit Performance monitoring counters.

Syntax (predefined)

.sreg .u64 %pm0_64;
.sreg .u64 %pm1_64;
.sreg .u64 %pm2_64;
.sreg .u64 %pm3_64;
.sreg .u64 %pm4_64;
.sreg .u64 %pm5_64;
.sreg .u64 %pm6_64;
.sreg .u64 %pm7_64;

Description

Special registers %pm064..%pm764 are unsigned 64-bit read-only performance monitor counters. Their behavior is currently undefined.

Notes

The lower 32bits of %pm064..%pm764 are identical to %pm0..%pm7.

PTX ISA Notes

%pm064..%pm764 introduced in PTX ISA version 4.0.

Target ISA Notes

%pm064..%pm764 require sm_50 or higher.

Examples

9.20. Driver-defined read-only registers:%envreg<32>.

Syntax (predefined)

.sreg .b32 %envreg<32>;

Description

A set of 32 pre-defined read-only registers used to capture execution environment of PTX program outside of PTX virtual machine. These registers are initialized by the driver prior to kernel launch and can contain cta-wide or grid-wide values.

Precise semantics of these registers is defined in the driver documentation.

PTX ISA Notes

Introduced in PTX ISA version 2.1.

Target ISA Notes

Supported on all target architectures.

Examples

$$mov.b32 %r1,%envreg0; // move envreg0 to %r1$$

9.21. Special Registers: %globaltimer, %globaltimer_lo, %globaltimer_hi

%globaltimer, %globaltimer_lo, %globaltimer_hi
%globaltimer
A predefined, 64-bit global nanosecond timer.
%globaltimer_lo
The lower 32-bits of %globaltimer.
%globaltimer_hi
The upper 32-bits of %globaltimer.

Syntax (predefined)

.sreg .u64 %globaltimer;
.sreg .u32 %globaltimer_lo, %globaltimer_hi;

Description

Special registers intended for use by NVIDIA tools. The behavior is target-specific and may change or be removed in future GPUs. When JIT-compiled to other targets, the value of these registers is unspecified.

PTX ISA Notes

Introduced in PTX ISA version 3.1.

Target ISA Notes

Requires target sm_30 or higher.

Examples

9.22. Special Registers: %total_smem_size

%total_smem_size
Total size of shared memory used by a CTA of a kernel.

Syntax (predefined)

.sreg .u32 %total_smem_size;

Description

A predefined, read-only special register initialized with total size of shared memory allocated (statically and dynamically) for the CTA of a kernel at launch time.

Size is returned in multiples of shared memory allocation unit size supported by target architecture.

Allocation unit values are as follows:

Target architecture Shared memory allocation unit size
sm_2x 128 bytes
sm3x, sm5x, sm_6x 256 bytes

PTX ISA Notes

Introduced in PTX ISA version 4.1.

Target ISA Notes

Requires sm_20 or higher.

Examples

9.23. Special Registers: %dynamic_smem_size

%dynamic_smem_size
Size of shared memory allocated dynamically at kernel launch.

Syntax (predefined)

.sreg .u32 %dynamic_smem_size;

Description

Size of shared memory allocated dynamically at kernel launch.

A predefined, read-only special register initialized with size of shared memory allocated dynamically for the CTA of a kernel at launch time.

PTX ISA Notes

Introduced in PTX ISA version 4.1.

Target ISA Notes

Requires sm_20 or higher.

Examples

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