icbt
Instruction Cache Block Touch - 7C 00 00 2C
icbt
Instruction Syntax
| Mnemonic | Format | Flags |
| icbt | CT,rA,rB | - |
Instruction Encoding
0
1
1
1
1
1
C
T
C
T
C
A
A
A
A
A
B
B
B
B
B
0
0
0
0
0
1
0
1
1
0
0
| Field | Bits | Description |
| Primary Opcode | 0-5 | 011111 (0x1F) |
| CT | 6-10 | Cache Touch hint |
| rA | 11-15 | Source register A |
| rB | 16-20 | Source register B |
| Reserved | 21 | 0 |
| XO | 22-30 | 000010110 (22) |
| Reserved | 31 | 0 |
Operation
if rA = 0 then EA ← 0 else EA ← (rA) EA ← EA + (rB) Touch instruction cache block containing EA with hint CT
The instruction cache block containing the effective address is touched, indicating to the processor that the program will likely access this instruction address in the near future. If rA is 0, the effective address is the contents of rB. Otherwise, the effective address is the sum of the contents of rA and rB.
Note: This is a hint instruction for performance optimization. The CT field provides additional cache touch hints. The instruction may be treated as a no-op by implementations that do not support instruction cache touch operations. This instruction is optional in the PowerPC architecture.
Affected Registers
None - This instruction does not affect any registers.
For more information on cache management see Section 2.1.1, "Cache Model," in the PowerPC Microprocessor Family: The Programming Environments manual.
Examples
Basic Instruction Cache Touch
# Touch instruction cache for upcoming code lis r3, hot_function@ha addi r3, r3, hot_function@l icbt 0, 0, r3 # Touch instruction cache block # ... some other work ... bl hot_function # Function likely already in cache
Branch Target Prefetching
# Prefetch likely branch targets before conditional branch lis r3, likely_path@ha addi r3, r3, likely_path@l icbt 0, 0, r3 # Touch likely branch target lis r4, unlikely_path@ha addi r4, r4, unlikely_path@l icbt 0, 0, r4 # Touch unlikely path too # Perform conditional logic cmpwi cr0, r5, 0 beq likely_path # Branch to prefetched code b unlikely_path # Branch to other prefetched code
Loop Unrolling Optimization
# Prefetch instruction cache for large unrolled loop
lis r3, unrolled_loop@ha
addi r3, r3, unrolled_loop@l
li r4, 0 # Offset
li r5, 1024 # Loop size
li r6, 32 # Cache line size
prefetch_loop:
add r7, r3, r4 # Calculate address
icbt 0, 0, r7 # Touch cache line
add r4, r4, r6 # Next cache line
cmpw r4, r5 # Check if done
blt prefetch_loop
# Now execute the unrolled loop with better cache performance
b unrolled_loop
Function Call Optimization
# Prefetch function entry points for call sequence
lis r3, func_table@ha
addi r3, r3, func_table@l
lwz r4, num_functions(r0)
li r5, 0
prefetch_functions:
lwzx r6, r3, r5 # Load function address
icbt 0, 0, r6 # Touch function entry point
addi r5, r5, 4 # Next function pointer
subi r4, r4, 1 # Decrement counter
cmpwi r4, 0
bne prefetch_functions
# Now call functions with better cache performance
li r5, 0
call_functions:
lwzx r6, r3, r5 # Load function address
mtctr r6 # Set up call
bctrl # Call function (likely in cache)
addi r5, r5, 4 # Next function
# ... continue calling ...
Interpreter Dispatch Table Optimization
# Prefetch interpreter dispatch targets lis r3, dispatch_table@ha addi r3, r3, dispatch_table@l lwz r4, bytecode_addr(r0) lbz r5, 0(r4) # Load opcode slwi r5, r5, 2 # Convert to word offset lwzx r6, r3, r5 # Load handler address icbt 0, 0, r6 # Prefetch handler code # Optionally prefetch next few opcodes lbz r7, 1(r4) # Next opcode slwi r7, r7, 2 lwzx r8, r3, r7 icbt 0, 0, r8 # Prefetch next handler # Execute current opcode mtctr r6 bctrl # Jump to handler
JIT Compiler Code Cache Warming
# Warm instruction cache for JIT compiled code
lis r3, jit_cache@ha
addi r3, r3, jit_cache@l
lwz r4, compiled_size(r0)
li r5, 0 # Current offset
li r6, 32 # Cache line size
warm_jit_cache:
add r7, r3, r5 # Current address
icbt 0, 0, r7 # Touch cache line
add r5, r5, r6 # Next cache line
cmpw r5, r4 # Check if done
blt warm_jit_cache
# Execute JIT compiled code with warmed cache
mtctr r3
bcctr 20, 0 # Execute JIT code
Game Engine Entity Update Loop
# Prefetch entity update functions for better performance
lis r3, entity_array@ha
addi r3, r3, entity_array@l
lwz r4, entity_count(r0)
li r5, 0 # Entity index
li r6, 32 # Entity struct size
entity_prefetch_loop:
# Calculate entity address
mullw r7, r5, r6
add r8, r3, r7 # Entity pointer
# Load function pointer and prefetch
lwz r9, update_func_offset(r8)
icbt 0, 0, r9 # Prefetch update function
addi r5, r5, 1 # Next entity
cmpw r5, r4 # Check if done
blt entity_prefetch_loop
# Now update entities with better cache performance
li r5, 0
entity_update_loop:
mullw r7, r5, r6
add r8, r3, r7 # Entity pointer
lwz r9, update_func_offset(r8)
mtctr r9
mr r3, r8 # Pass entity as parameter
bctrl # Call update function
addi r5, r5, 1
cmpw r5, r4
blt entity_update_loop
Database Query Execution Plan
# Prefetch query operator code for execution plan
lis r3, query_plan@ha
addi r3, r3, query_plan@l
lwz r4, num_operators(r0)
li r5, 0 # Operator index
prefetch_operators:
# Each operator has function pointer at offset 0
slwi r6, r5, 3 # 8 bytes per operator struct
add r7, r3, r6 # Operator struct address
lwz r8, 0(r7) # Load operator function
icbt 0, 0, r8 # Prefetch operator code
addi r5, r5, 1 # Next operator
cmpw r5, r4 # Check if done
bne prefetch_operators
# Execute query plan with prefetched operators
li r5, 0
execute_operators:
slwi r6, r5, 3
add r7, r3, r6
lwz r8, 0(r7) # Load operator function
lwz r9, 4(r7) # Load operator data
mtctr r8
mr r3, r9 # Pass data as parameter
bctrl # Execute operator
addi r5, r5, 1
cmpw r5, r4
blt execute_operators
State Machine Transition Prefetching
# Prefetch state machine transition handlers lis r3, state_machine@ha addi r3, r3, state_machine@l lwz r4, current_state(r3) lwz r5, event_code(r0) # Calculate transition table entry slwi r6, r4, 8 # 256 events per state add r6, r6, r5 # Add event code slwi r6, r6, 2 # 4 bytes per pointer add r7, r3, r6 # Transition table entry lwz r8, transition_table_offset(r7) icbt 0, 0, r8 # Prefetch transition handler # Also prefetch likely next states addi r9, r5, 1 # Next event cmpwi r9, 256 bge skip_next_prefetch slwi r10, r4, 8 add r10, r10, r9 slwi r10, r10, 2 add r11, r3, r10 lwz r12, transition_table_offset(r11) icbt 0, 0, r12 # Prefetch next likely transition skip_next_prefetch: # Execute current transition mtctr r8 mr r3, r5 # Pass event as parameter bctrl # Execute transition handler
Packet Processing Pipeline
# Network packet processing with function prefetching lis r3, packet_buffer@ha addi r3, r3, packet_buffer@l lbz r4, packet_type(r3) # Load packet type lis r5, handler_table@ha addi r5, r5, handler_table@l slwi r6, r4, 2 # 4 bytes per handler pointer lwzx r7, r5, r6 # Load handler address icbt 0, 0, r7 # Prefetch packet handler # Prefetch common follow-up handlers li r8, PKT_TYPE_ACK # Common follow-up packet type slwi r9, r8, 2 lwzx r10, r5, r9 icbt 0, 0, r10 # Prefetch ACK handler # Process current packet mtctr r7 mr r3, r3 # Pass packet buffer bctrl # Process packet
Graphics Shader Pipeline Optimization
# Prefetch shader programs for rendering pipeline lis r3, shader_cache@ha addi r3, r3, shader_cache@l lwz r4, active_material(r0) lwz r5, vertex_shader_offset(r4) add r6, r3, r5 # Vertex shader address icbt 0, 0, r6 # Prefetch vertex shader lwz r7, fragment_shader_offset(r4) add r8, r3, r7 # Fragment shader address icbt 0, 0, r8 # Prefetch fragment shader # Prefetch geometry shader if present lwz r9, geometry_shader_offset(r4) cmpwi r9, 0 beq skip_geometry_prefetch add r10, r3, r9 icbt 0, 0, r10 # Prefetch geometry shader skip_geometry_prefetch: # Execute rendering pipeline with prefetched shaders mtctr r6 bctrl # Execute vertex shader mtctr r8 bctrl # Execute fragment shader