http://llvm.org/bugs/show_bug.cgi?id=20568
Bug ID: 20568
Summary: failure to remove trivially dead loop
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
From
http://stackoverflow.com/questions/25151508/why-dont-modern-c-compilers-optimize-away-simple-loops-like-this-clang-msv
LLVM does not remove the loops in this code:
#include <stdio.h>
#include <time.h>
static int const N = 0x8000;
int main()
{
clock_t const start = clock();
for (int i = 0; i < N; ++i)
{
int a[N]; // Never used outside of this block, but not optimized
away
for (int j = 0; j < N; ++j)
{
++a[j]; // This is undefined behavior too, but Clang doesn't see
it
}
}
clock_t const finish = clock();
fprintf(stderr, "%u ms\n",
static_cast<unsigned int>((finish - start) * 1000 / CLOCKS_PER_SEC));
return 0;
}
(See also PR13050; this seems like a different problem, though.)
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