[Bug c/101432] New: NULL dereferences aren't assumed to be unreachable even with -fdelete-null-pointer-checks -fno-isolate-erroneous-paths-dereference

[Bug c/101432] New: NULL dereferences aren't assumed to be unreachable even with -fdelete-null-pointer-checks -fno-isolate-erroneous-paths-dereference

[josephcsible at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101432

            Bug ID: 101432
           Summary: NULL dereferences aren't assumed to be unreachable
                    even with -fdelete-null-pointer-checks
                    -fno-isolate-erroneous-paths-dereference
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: josephcsible at gmail dot com
  Target Milestone: ---
            Target: x86_64-linux-gnu

Consider this C code:

#include <stddef.h>
int f(_Bool x) {
    if(x) {
        int *null = NULL;
        return *null;
    } else {
        return 42;
    }
}

I want this assembly:

f:
        movl    $42, %eax
        ret

But even with -O3 -fdelete-null-pointer-checks
-fno-isolate-erroneous-paths-dereference, I get this assembly:

f:
        movl    $42, %eax
        testb   %dil, %dil
        je      .L1
        movl    0, %eax
.L1:
        ret

[Bug tree-optimization/101432] NULL dereferences aren't assumed to be unreachable even with -fdelete-null-pointer-checks -fno-isolate-erroneous-paths-dereference

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101432

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
             Status|UNCONFIRMED                 |NEW
   Last reconfirmed|                            |2021-07-13

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
-fisolate-erroneous-paths-dereference is the only one that "elides" the null
dereference but it doesn't currently have a way to optimistically remove it,
instead it places a trap at the path entry unconditionally.  Thus you get

f:
.LFB0:
        .cfi_startproc
        testb   %dil, %dil
        je      .L2
        movl    0, %eax
        ud2
        .p2align 4,,10
        .p2align 3
.L2:
        movl    $42, %eax
        ret

there are duplicate enhancement requests for an option to control this
behavior.  The current default behavior is on purpose.