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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - [X86_64] Excess movs in XXH64 loop"
href="https://bugs.llvm.org/show_bug.cgi?id=42545">42545</a>
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<th>Summary</th>
<td>[X86_64] Excess movs in XXH64 loop
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>All
</td>
</tr>
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<th>OS</th>
<td>All
</td>
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<th>Status</th>
<td>NEW
</td>
</tr>
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<th>Severity</th>
<td>enhancement
</td>
</tr>
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<th>Priority</th>
<td>P
</td>
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<th>Component</th>
<td>Backend: X86
</td>
</tr>
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
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<th>Reporter</th>
<td>husseydevin@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>craig.topper@gmail.com, llvm-bugs@lists.llvm.org, llvm-dev@redking.me.uk, spatel+llvm@rotateright.com
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<pre>Clang seems to generate extra mov instructions in the XXH64 loop, significantly
hampering performance.
#include <stdint.h>
#include <stddef.h>
static const uint64_t PRIME64_1 = 11400714785074694791ULL;
uint64_t XXH64_mini(const uint64_t* input, size_t len)
{
const uint64_t *limit = input + len;
uint64_t v1 = 0;
do {
v1 += *input++ * PRIME64_1;
v1 = (v1 << 31) | (v1 >> (64 - 31)); // rotate
v1 *= PRIME64_1;
} while (input<=limit);
return v1;
}
This is a simplified version of XXH64's loop.
With -O3, I would expect something like this:
uint64_t val;
val = *input;
val *= PRIME64_1;
acc += acc;
acc = (acc << 31) | (acc >> (64-31));
acc *= PRIME64_1;
XXH64_mini:
movabs rcx, -7046029288634856825
lea rdx, [rdi + 8*rsi]
xor eax, eax
.LBB0_1:
mov rsi, qword ptr [rdi]
imul rsi, rcx
add rdi, 8
add rax, rsi
rol rax, 31
imul rax, rcx
cmp rdi, rdx
jbe .LBB0_1
ret
However, Clang swaps the add instruction's operands, resulting in an extra mov:
uint64_t val;
val = *input;
val *= PRIME64_1;
val += acc;
val = (val << 31) | (val >> (64-31));
acc = val;
acc *= PRIME64_1;
XXH64_mini:
movabs rcx, -7046029288634856825
lea rdx, [rdi + 8*rsi]
xor eax, eax
.LBB0_1:
mov rsi, qword ptr [rdi]
imul rsi, rcx
add rdi, 8
add rsi, rax # <<<
rol rsi, 31 # <<<
mov rax, rsi # <<<
imul rax, rcx
cmp rdi, rdx
jbe .LBB0_1
ret
GCC and MSVC both seem to emit the proper code, as seen here:
<a href="https://godbolt.org/z/wu7jDY">https://godbolt.org/z/wu7jDY</a>
It appears that all versions of LLVM do this.
I can't seem to figure out how to further simplify this.</pre>
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