[llvm-bugs] [Bug 49205] New: compile-time explosion with assumes at -O1 to -Os

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=49205

            Bug ID: 49205
           Summary: compile-time explosion with assumes at -O1 to -Os
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: haoxintu at gmail.com
                CC: llvm-bugs at lists.llvm.org

Hi, all.

This code, s.c, is a valid program but makes Clang hung on at compile time at
-O1 to -Os.

$cat s.c
int a;
void b() {
  int *c = &a;
  int *d = &a;
  int **e = &d;
  for (*c = 7; c <= 2; *c += 1) {
    int **f = d;
    *c = 0;
    **e /= f ? (c ? 10 : **f) % (*c &= 6) : 0;
  }
  *d = (*d -= *c & **e != 4) && (4 != a) - **e & d == c;
  int g = &d;
  g++;
}

$time clang -c -w -O0 s.c

real    0m0.021s
user    0m0.011s
sys     0m0.010s



$clang -c -w -O1 test.cc
//endless compiling (the same as -O2,-O3, and -Os options)

$clang -v
clang version 13.0.0 (https://github.com/llvm/llvm-project
22f00f61dd5483a9fdaed3b7592d392c23b3646a)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir:
/home/haoxin/haoxin-data/dut-research/compilers/llvm-project/build-20210216/bin
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.5.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8
Selected GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8
Candidate multilib: .;@m64
Selected multilib: .;@m64

This problem only occurs in clang-trunk and clang-11.0.0 or clang-11.0.1, while
other released versions from clang-10 downwards perform well in this case.


Thanks.
Haoxin

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