Miscompilation with libstdc++'s std::optional<int> and -O1

[cadubentzen]

https://godbolt.org/z/KjEreWf57

With Clang 18.1 and libstdc++, we get different behavior when compared to Clang 17 in the code below.
EDIT: requires at least -O1.

#include <optional>
#include <iostream>

// defined in a separate compilation unit
int takeIntRefAndReturn0(int&);

std::optional<int> shouldReturnEmptyOptional() {
  int v = 5;
  if (takeIntRefAndReturn0(v))
    return v;
  return std::nullopt;
}

int main() {
  auto opt = shouldReturnEmptyOptional();
  if (opt && *opt > 0) {
    std::cout << "SHOULD NOT BE PRINTED: *opt = " << *opt << std::endl;
    return 1;
  }
  std::cout << "SHOULD BE PRINTED" << std::endl;
}

With Clang 17, we get SHOULD BE PRINTED, while with with Clang 18.1 we get SHOULD NOT BE PRINTED: *opt = 5.

With git-bisect, I found that this is caused by 060de41.

An isomorphic example to reproduce this (https://godbolt.org/z/9PsjY17sT):

int takeIntRefReturn0(int &);
void assertNotReached(int);

static bool logicalAnd(bool a, bool b) { return a && b; }

int main() {
  int v4;
  bool v3;
  {
    int v1 = 5;
    int v2 = takeIntRefReturn0(v1);
    v3 = v2 != 0;
    if (v3)
      v4 = v1;
  }
  // Move  to a function so that && is not short cutted.
  // v4 will be undefined if v2 == 0, but v3 is false, so the branch shouldn't be entered.
  if (logicalAnd(v3, v4 > 0))
    assertNotReached(v4);

  return 0;
}

Note in the generated LLVM IR that

%6 = icmp sgt i32 %5, 0
%7 = and i1 %3, %6
br i1 %7, label %8, label %9

was reduced to only

%6 = icmp sgt i32 %5, 0
br i1 %6, label %7, label %8

[dtcxzyw]

Reproducer: https://alive2.llvm.org/ce/z/p4ZGxe

; bin/opt -passes=instcombine test.ll -S
define i1 @src(i32 noundef %x, i32 %y) {
  %3 = icmp ne i32 %x, 0
  %5 = select i1 %3, i32 %y, i32 undef
  %6 = icmp sgt i32 %5, 0
  %7 = and i1 %3, %6
  br i1 %7, label %if.then, label %if.else

if.then:
  call void @use(i32 noundef %5)
  ret i1 true

if.else:
  ret i1 false
}

define i1 @tgt(i32 noundef %x, i32 %y) {
  %.not = icmp eq i32 %x, 0
  %1 = select i1 %.not, i32 undef, i32 %y
  %2 = icmp sgt i32 %1, 0
  br i1 %2, label %if.then, label %if.else

if.then:                                          ; preds = %0
  call void @use(i32 noundef %1)
  ret i1 true

if.else:                                          ; preds = %0
  ret i1 false
}

declare void @use(i32 %x)

cc @nikic

[nikic]

A bit simpler using just instsimplify: https://alive2.llvm.org/ce/z/tnkf6A

[nikic]

I expect the fix here is to perform replacements with Q.getWithoutUndef(). But probably not just here, but also in other places using simplifyWithOpReplaced(). (Possibly making it always use that mode.)

[cadubentzen]

Thanks a lot for fixing this so quickly! I wonder if this should be included in 18.1.9?

[nikic]

The LLVM 18 branch is already closed, there will be no further releases.

[thesamesam]

I'll add the milestone anyway as it's easier to track for downstream backports then.