https://bugs.llvm.org/show_bug.cgi?id=36135
Bug ID: 36135
Summary: Missed optimization: loop-invariant strlen() not
hoisted out of loop
Product: compiler-rt
Version: 5.0
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: compiler-rt
Assignee: unassignedb...@nondot.org
Reporter: eyal...@technion.ac.il
CC: llvm-bugs@lists.llvm.org
Consider the following code:
#include <string.h>
void bar(char c);
void foo(const char* __restrict__ ss)
{
for (int i = 0; i < strlen(ss); ++i)
{
bar(*ss);
}
}
To my understanding, the fact that ss is __restrict__ed, combined with the fact
that it isn't written through (or that it's a const pointer) should be
sufficient to allow the compiler to assume the memory accessible via ss remains
constant, and thus assume that strlen(ss) will return the same value for all
loop iterations.
But - that's not what happens (with clang 5.0):
.LBB0_2: # =>This Inner Loop Header: Depth=1
mov edi, ebp
call bar(char)
inc rbx
mov rdi, r14
call strlen
cmp rax, rbx
ja .LBB0_2
(obtained with https://godbolt.org/g/vdGSBe )
Now, I'm no compiler expert, so maybe there are considerations I'm ignoring,
but it seems to me the compiler should be able to hoist the heavier code up
above the loop.
Cf. https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84083 and
https://stackoverflow.com/q/48482003/1593077
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