andygrove opened a new pull request, #6419:
URL: https://github.com/apache/arrow-rs/pull/6419
# Which issue does this PR close?
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N/A
# Rationale for this change
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the issue then this section is not needed.
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Small performance optimization.
# What changes are included in this PR?
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I noticed two areas of overhead in the current approach to verifying decimal
precision.
1. In the event of an overflow, we are building a formatted string for an
error message, which is then discarded in some cases, so we should avoid that
cost
2. The range check code introduces a memcpy which can be avoided
I tested the following variations of the decimal precision check in Rust
playground.
```rust
fn validate_decimal_precision1(value: i128, precision: u8) -> bool {
if precision > DECIMAL128_MAX_PRECISION {
return false;
}
let idx = usize::from(precision) - 1;
value >= MIN_DECIMAL_FOR_EACH_PRECISION[idx] && value <=
MAX_DECIMAL_FOR_EACH_PRECISION[idx]
}
// based on arrow-rs version
fn validate_decimal_precision2(value: i128, precision: u8) -> bool {
if precision > DECIMAL128_MAX_PRECISION {
return false;
}
let max = MAX_DECIMAL_FOR_EACH_PRECISION[usize::from(precision) - 1];
let min = MIN_DECIMAL_FOR_EACH_PRECISION[usize::from(precision) - 1];
if value > max {
false
} else if value < min {
false
} else {
true
}
}
```
`validate_decimal_precision1` avoids a `memcpy` that appears in
`validate_decimal_precision2`:
```
playground::validate_decimal_precision1:
subq $1304, %rsp
movb %dl, %al
movb %al, 23(%rsp)
movq %rsi, 24(%rsp)
movq %rdi, 32(%rsp)
movq %rdi, 1264(%rsp)
movq %rsi, 1272(%rsp)
movb %al, 1287(%rsp)
cmpb $38, %al
ja .LBB9_2
movb 23(%rsp), %al
movb %al, 1303(%rsp)
movzbl %al, %eax
movq %rax, %rcx
subq $1, %rcx
movq %rcx, 8(%rsp)
cmpq $1, %rax
jb .LBB9_4
jmp .LBB9_3
playground::validate_decimal_precision2:
subq $1368, %rsp
movb %dl, %al
movb %al, 55(%rsp)
movq %rsi, 56(%rsp)
movq %rdi, 64(%rsp)
movq %rdi, 1296(%rsp)
movq %rsi, 1304(%rsp)
movb %al, 1327(%rsp)
cmpb $38, %al
ja .LBB10_2
leaq 80(%rsp), %rdi
leaq .L__unnamed_5(%rip), %rsi
movl $608, %edx
callq memcpy@PLT <----- MEMCPY HERE
movb 55(%rsp), %al
movb %al, 1367(%rsp)
movzbl %al, %eax
movq %rax, %rcx
subq $1, %rcx
movq %rcx, 40(%rsp)
cmpq $1, %rax
jb .LBB10_4
jmp .LBB10_3
```
# Are there any user-facing changes?
No
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