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1–14 9.1 Altera Corporation
Nios II C2H Compiler User Guide November 2009
C Code Appropriate for Hardware Acceleration
Example 1–1. Checksum Calculation
u16_t standard_chksum(void *dataptr, int len)
{
u32_t acc;
/* Checksum loop: iterate over all data in buffer */
for(acc = 0; len > 1; len -= 2)
{
acc += *(u16_t *)dataptr;
dataptr = (void *)((u16_t *)dataptr + 1);
}
/* Handle odd buffer lengths */
if (len == 1)
{
acc += htons((u16_t)((*(u8_t *)dataptr)&0xff)<< 8);
}
/* Modify result for IP stack needs */
acc = (acc >> 16) + (acc & 0xffffUL);
if ((acc & 0xffff0000) != 0)
{
acc = (acc >> 16) + (acc & 0xffffUL);
}
return (u16_t)acc;
}
Accelerating this function could have a significant impact on execution
time, especially the amount of time spent in the for loop. The remaining
code executes once per call to format the result and check boundary cases.
Accelerating the code outside the loop has little benefit, unless the entire
standard_chksum() function is a called from another function that is
also a good acceleration candidate. The most efficient hardware
accelerator for this code would replace only the for loop. To accelerate
the for loop only, you need to refactor the code to isolate the loop in a
separate function.
Poor Acceleration Candidates
Accelerating some code can have negative performance impacts, or can
unacceptably increase resource utilization, or both. Use the following
guidelines to identify functions not to accelerate:
■ Code that contains many data or control dependencies must perform
many sequential operations, and is a poor candidate for acceleration.
A large number of dependencies makes it difficult for the
C2H Compiler to fully optimize loops. Processors are designed to
perform such operations efficiently.
- Nios II C2H Compiler 1
- User Guide 1
- UG-N2C2HCMPLR-1.6 2
- Contents 3
- Chapter 6. Pragma Reference 4
- 1. Introduction to the 7
- Target Audience 8
- Introduction 8
- Altera Corporation 9.1 1–5 11
- C2H Compiler 12
- Concepts 12
- Hardware and Software 13
- 1–8 9.1 Altera Corporation 14
- C2H Compiler Concepts 14
- System Architecture 15
- Appropriate for 19
- Hardware 19
- Acceleration 19
- Poor Acceleration Candidates 20
- Next Steps 22
- 2. Getting Started Tutorial 23
- Typical Design 24
- OpenCore Plus Evaluation 25
- Tutorial 26
- Altera Corporation 9.1 2–5 27
- Getting Started Tutorial 27
- Create the Software Project 28
- Altera Corporation 9.1 2–7 29
- 2–8 9.1 Altera Corporation 30
- Altera Corporation 9.1 2–9 31
- Rebuild the Project 32
- Remove the Accelerator 37
- 3. C-to-Hardware Mapping 41
- Reference 41
- Assignments 42
- >> 44
- << 44
- Iteration Statements 45
- Selection Statements 46
- Altera Corporation 9.1 3–7 47
- Conditional Operator ?: 48
- Altera Corporation 9.1 3–9 49
- Subfunction Calls 51
- Variable 53
- Declarations 53
- Scalar Variables 54
- Accesses 55
- Memory Accesses 58
- Volatile Type Qualifier 59
- Address Computation 61
- Data Computation 62
- Master-Slave Connections 63
- Array Subscript Operator 66
- Structure and Union Operators 68
- Scheduling 70
- Data Dependencies 71
- Pointer Aliasing 72
- int offset_b) 75
- my_ptr[offset_a] = a + 7; 75
- my_ptrb[offset_b] = b + 8; 75
- Read Operations with Latency 77
- Stalling 79
- Loop Pipelining 82
- Loop-Carried Dependencies 84
- Subfunction Pipelining 89
- Resource 91
- Resource Sharing 92
- 4. Understanding the C2H 95
- Configuration Meaning 96
- Altera Corporation 9.1 4–5 99
- 4–6 9.1 Altera Corporation 100
- Resources 100
- Altera Corporation 9.1 4–7 101
- Understanding the C2H View 101
- 4–8 9.1 Altera Corporation 102
- Altera Corporation 9.1 4–9 103
- Performance 104
- Loop Latency 105
- Scheduling Information 108
- Further Reading 113
- 5. Accelerating Code Using 115
- Tools Command Line 115
- Altera Corporation 9.1 5–3 117
- 5–4 9.1 Altera Corporation 118
- C2H Performance Metrics 118
- 6. Pragma Reference 119
- Reducing Arbitration Logic 120
- Flow Control 121
- Interrupt 122
- Unshare Pointer 123
- 6–6 9.1 Altera Corporation 124
- Unshare Pointer Pragma 124
- 7. ANSI C Compliance and 125
- Restrictions 125
- 7–2 9.1 Altera Corporation 126
- Language 126
- Expressions 127
- Functions 128
- Altera Corporation 9.1 7–5 129
- 7–6 9.1 Altera Corporation 130
- Altera Corporation 9.1 7–7 131
- Goto (Section 6.8.6.1) 132
- Other Restrictions 134
- Additional Information 135
- Revision History 136
- How to Contact 137
- Typographic 137
- Conventions 137
- Visual Cue Meaning 138
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