[llvm-bugs] [Bug 33646] New: ThinLTO doesn't apply interprocedural DeadArgumentElimination optimization

via llvm-bugs llvm-bugs at lists.llvm.org
Thu Jun 29 10:33:24 PDT 2017


            Bug ID: 33646
           Summary: ThinLTO doesn't apply interprocedural
                    DeadArgumentElimination optimization
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Interprocedural Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: charles.saternos at gmail.com
                CC: llvm-bugs at lists.llvm.org

LTO will apply the deadargelim pass, but ThinLTO can't apply this optimization
right now.

$ cat a.cc
int f(const int *x, int j) {
    int k = *x;
    return k + 1;

int g(int *x) {
    return *x;
$ cat b.cc
#include <stdio.h>

extern int f(const int *x, int j);
extern int g(int *x);

int main(void) {
  int x = 3;
  printf("%d\n", f(&x, x));
  printf("%d\n", x);

$ ../build/bin/clang++ a.cc b.cc -O3 --for-linker=-mllvm --for-linker=-stats
-flto=thin -fuse-ld=lld -o thin &>&1 | grep deadargelim
# nothing
$ ../build/bin/clang++ a.cc b.cc -O3 --for-linker=-mllvm --for-linker=-stats
-flto -fuse-ld=lld -o lto &>&1 | grep deadargelim
   1 deadargelim           - Number of unread args removed
   1 deadargelim           - Number of unused return values removed

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