https://llvm.org/bugs/show_bug.cgi?id=24678
Bug ID: 24678
Summary: are overlapping memory accesses optimal?
Product: libraries
Version: trunk
Hardware: PC
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Common Code Generator Code
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
I'm not sure if this is a performance bug, but I'm filing it for further review
based on the discussion in D12543:
http://reviews.llvm.org/D12543
The code in SelectionDAG's FindOptimalMemOpLowering() can generate overlapping
accesses when unaligned memops are specified as fast, but it's not clear if
overlapping is good for performance on all targets.
Example:
$ cat copy13bytes.c
#include <string.h>
void foo(char *a, char *b) {
memcpy(a, b, 13);
}
$ clang copy13bytes.c -S -o - -O2
...
movq (%rsi), %rax
movq 5(%rsi), %rcx
movq %rcx, 5(%rdi)
movq %rax, (%rdi)
$ gcc copy13bytes.c -S -o - -O2
...
movq (%rsi), %rax
movq %rax, (%rdi)
movl 8(%rsi), %eax
movl %eax, 8(%rdi)
movzbl 12(%rsi), %eax
movb %al, 12(%rdi)
Note that any load/store in either case may be misaligned (and in the clang
case, at least one pair of the ops are guaranteed to be misaligned), but LLVM
chooses overlapping ops to reduce the instruction count.
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