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'itype' names follow ARC-defined values (10, 11)
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mipsrobert committed Apr 1, 2024
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65 changes: 36 additions & 29 deletions docs/RISC-V-N-Trace.adoc
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[[header]]
:description: RISC-V N-Trace (Nexus-based Trace)
:company: RISC-V.org
:revdate: Mar 30, 2024
:revdate: Apr 01, 2024
:revnumber: 1.0.0_rc26
:revremark: Stable state (first step of 'itype' unification)
:revremark: Stable state (before SYNC clarifications)
:url-riscv: http://riscv.org
:doctype: book
:preface-title: Preamble
Expand Down Expand Up @@ -52,10 +52,8 @@ PDF generated on: {localdatetime}

=== Version 1.0.0_rc26
* 2024-04-01
** First step of 'itype' unification (inferable/uninferable changed to direct/indirect)
** Renamed '6' to **Indirect jump (with and without linkage)**.
** Visual aspects of table[s] fixed.
** Next step will include split of some 14/15 into 10/11 names and fix conditions.
** 'itype' clarifications done.
** TODO: SYNC clarification (email exchange with Ved).

[Preface]
== Copyright and license information
Expand Down Expand Up @@ -212,35 +210,37 @@ The table below provides a detailed mapping of causes for terminating an instruc
|EBREAK, ECALL, C.EBREAK|An exception trap that occurred following the final retired instruction in the block due to these instructions. These instructions do not retire.
|1 = Exception
|Interrupted instruction|An interrupt trap occurred following the final retired instruction in the block|2 = Interrupt
|MRET, SRET| Return from an exception of interrupt handler. |3 = Trap return
|<<itype_branch,Conditional branch>>|Not-taken |4 = Not-taken branch
|<<itype_branch,Conditional branch>>|Taken |5 = Taken branch
|MRET, SRET| Return from an exception or interrupt handler. |3 = Trap return
|<<itype_branch,Conditional branch>>|Not-taken direct, conditional branch.|4 = Not-taken branch
|<<itype_branch,Conditional branch>>|Taken direct, conditional branch.|5 = Taken branch
|Any other instruction|All other instructions that are not directly listed in this table.
|0 = No special type
3+|*Values of itype (<<itype_3_4,3-bit>>) (without <<Implicit Return Optimization,Implicit Return Optimization>>*)
|JAL rd |Any direct jump/call. |0 = No special type
|JALR |Any indirect jump/call. |6 = Indirect jump (with or without linkage)
|JALR rd, rs |Any indirect jump/call. |6 = Indirect jump (with or without linkage)
|C.J or C.JAL |C extension has direct jump/calls only. |0 = No special type
|CM.JT |Defined by <<zcmt,Zcmt>> extension. |0 = No special type
|CM.JALT |Defined by <<zcmt,Zcmt>> extension. |0 = No special type
|CM.POPRET* |Defined by **Zcmp** extension. |6 = Indirect jump (with or without linkage)
3+|*Values of itype (<<itype_3_4,4-bit>>) (needed for <<Implicit Return Optimization,Implicit Return Optimization>>*). <<link,link>> means *x1* or *x5*.
|JAL rd |rd = `link` |9 = Direct call
| |rd != `link` |15 = Other direct jump
|JALR rd, rs1 |rd = `link` and rs1 != `link` |8 = Indirect call
| |rd = `link` and rs1 = `link` and rd != rs1 |12 = Co-routine swap
| |rd = `link` and rs1 = `link` and rd = rs1 |8 = Indirect call
| |rd != `link` and rs1 = `link` |13 = Function return
| |rd != `link` and rs1 != `link` |14 = Other indirect jump
| |rd = *x0* |11 = Direct jump (without linkage)
| |rd != `link` and rd != *x0* |15 = Other direct jump (with linkage)
|JALR rd, rs |rd = `link` and rs != `link` |8 = Indirect call
| |rd = `link` and rs = `link` and rd = rs |8 = Indirect call
| |rd = `link` and rs = `link` and rd != rs |12 = Co-routine swap
| |rd != `link` and rs = `link` |13 = Function return
| |rd = *x0* and rs != `link` |10 = Indirect jump (without linkage)
| |rd != `link` and rd != *x0* and rs != `link` |14 = Other indirect jump (with linkage)
|C.JAL |Expands to `JAL x1, offset` |9 = Direct call
|C.JALR rs1 |rs1 = x5 |12 = Co-routine swap
| |rs1 != x5 |8 = Indirect call
|C.JR rs1 |rs1 = `link` |13 = Return
| |rs1 != `link` |14 = Other indirect jump
|C.J |No registers, only offset. |15 = Other direct jump
|CM.JT |Defined by <<zcmt,Zcmt>> extension. |15 = Other direct jump
|C.JALR rs |rs = *x5* |12 = Co-routine swap
| |rs != *x5* |8 = Indirect call
|C.JR rs |rs = `link` |13 = Function return
| |rs != `link` |10 = Indirect jump (without linkage)
|C.J |Expands to `JAL x0, offset` |11 = Direct jump (without linkage)
|CM.JT |Defined by <<zcmt,Zcmt>> extension. |11 = Direct jump (without linkage)
|CM.JALT |Defined by <<zcmt,Zcmt>> extension. |9 = Direct call
|CM.POPRET* |Defined by **Zcmp** extension. |13 = Return
|CM.POPRET* |Defined by **Zcmp** extension. |13 = Function return
|======================================================================================================

[[itype_branch]]
Expand Down Expand Up @@ -300,26 +300,33 @@ When the *itype* input of ingress port is 4-bit wide, the Indirect jump (with or
|#|itype|Encoder Action|Stack Action
|8|Indirect call|Update <<field_I-CNT,I-CNT>> field. Emit Indirect Branch message with <<field_B-TYPE,B-TYPE>>=0|Push
|9|Direct call|Only update <<field_I-CNT,I-CNT>> field.|Push
|10|Reserved||-
|11|Reserved||-
|10|Indirect jump (without linkage)|Update <<field_I-CNT,I-CNT>> field. +
Emit Indirect Branch message with <<field_B-TYPE,B-TYPE>>=0. +
<<same_handling,Same handing>> as *itype=14*|-
|11|Direct jump (without linkage)|Only update <<field_I-CNT,I-CNT>> field. +
<<same_handling,Same handing>> as *itype=15*|-
|12|Co-routine swap|Update <<field_I-CNT,I-CNT>> field. +
If Pop does not returns the same address as PC at next valid ingress port cycle, emit Indirect Branch message with <<field_B-TYPE,B-TYPE>>=0.|Pop,Push
|13|Return|Update <<field_I-CNT,I-CNT>> field. +
If Pop does not returns the same address as PC at next valid ingress port cycle, emit Indirect Branch message with <<field_B-TYPE,B-TYPE>>=0.
|Pop
|14|Other indirect jump|Update <<field_I-CNT,I-CNT>> field. +
Emit Indirect Branch message with <<field_B-TYPE,B-TYPE>>=0.|-
|15|Other direct jump|Only update <<field_I-CNT,I-CNT>> field.|-
|14|Other indirect jump (with linkage)|Update <<field_I-CNT,I-CNT>> field. +
Emit Indirect Branch message with <<field_B-TYPE,B-TYPE>>=0. +
<<same_handling,Same handing>> as *itype=10*|-
|15|Other direct jump (with linkage)|Only update <<field_I-CNT,I-CNT>> field. +
<<same_handling,Same handing>> as *itype=11*|-
|======================================================================================================

[[same_handling]]
IMPORTANT: N-Trace messages do not differentiate instructions classified as *... jump (with linkage)* and *... jump (without linkage)*, so both N-Trace ingress ports and N-Trace encoders implementations may ignore differences between *with/without linkage* values.

If optional <<trTeInstEnAllJumps,trTeInstEnAllJumps>> bit is set, trace ingress port is required to report *itype*=5 (Taken branch) for all direct unconditional jumps, which are normally reported as *itype* = 0 or 15.

IMPORTANT: The N-Trace encoder does not require *cause* and *tval* ingress port
signals, which are valid only for exceptions and interrupts, as these
details are not reported in N-Trace messages. Instead, N-Trace solely
provides the address of the exception or interrupt handler


NOTE: Since almost every ingress port cycle updates I-CNT, there is a possibility
of overflow. For more information, see <<I-CNT Details, I-CNT Details>> chapter regarding
I-CNT management and overflow handling.
Expand Down

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