[cfe-dev] Evaluate constant condition of if statement has no effect

[Frozen via cfe-dev]

Simple Example:

int main()
{
  int x;
  if (x || 1) {}
}


Clang can not evaluate this condition and will emit IR like this:


define signext i32 @main() #0 {
entry:
  %retval = alloca i32, align 4
  %x = alloca i32, align 4
  store i32 0, i32* %retval, align 4
  %0 = load i32, i32* %x, align 4
  %tobool = icmp ne i32 %0, 0
  br i1 %tobool, label %if.then, label %lor.lhs.false


lor.lhs.false:                                    ; preds = %entry
  br i1 true, label %if.then, label %if.end


if.then:                                          ; preds = %lor.lhs.false, %entry
  br label %if.end


if.end:                                           ; preds = %if.then, %lor.lhs.false
  %1 = load i32, i32* %retval, align 4
  ret i32 %1
}


However, when we swap the position of LHS and RHS(i.e. if (1 || x), Clang can recognize it:


define signext i32 @main() #0 {
entry:
  %x = alloca i32, align 4
  ret i32 0
}

I also find the root issue and propose one potential solution in comment 2 of this link: https://bugs.llvm.org/show_bug.cgi?id=34229

Any idea?

Thanks in advance.
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