Bug 28306

Summary: const / pure call with ignored argument emitted.
Product: gcc Reporter: Jorn Wolfgang Rennecke <amylaar>
Component: cAssignee: Not yet assigned to anyone <unassigned>
Status: NEW ---    
Severity: normal CC: dimhen, gcc-bugs, rguenth
Priority: P3 Keywords: missed-optimization
Version: 4.2.0   
Target Milestone: ---   
Host: Target:
Build: Known to work:
Known to fail: Last reconfirmed: 2019-03-05 00:00:00
Bug Depends on:    
Bug Blocks: 29842, 28289    
Attachments: test case

Description Jorn Wolfgang Rennecke 2006-07-07 16:52:16 UTC 
There is code in the expanders which is supposed to avoid emitting calls
to pure functions which have their result ignored, but it doesn't appear to
work when the function called is represented as a COND_EXPR.
Comment 1 Jorn Wolfgang Rennecke 2006-07-07 16:54:27 UTC 
Created attachment 11850 [details]
test case

The available source code for the functions allows the compiler to
see that the called functions are const.
This should be sufficient for the compiler to figure out the call is not needed.
Likewise, the pure attributes should also be sufficient to figure this out.
Comment 2 Richard Biener 2006-07-07 17:30:13 UTC 
Confirmed.
Comment 3 Richard Biener 2008-01-13 18:22:35 UTC 
This works for 'const' functions because they stick on the function type as well.
The attributes are only on the DECL and do not get merged and transfered to the
resulting function type of the COND_EXPR.

So this again asks for streamlining attributes on types vs. decls.
Comment 4 Steven Bosscher 2019-03-05 09:01:27 UTC 
GCC trunk today on x86-64 produces:

gen_x86_64_shrd(int):
        xorl    %eax, %eax
        ret
ok(int):
        movl    $1, %eax
        ret
ix86_split_ashr(int):
        testl   %edi, %edi
        movl    $ok(int), %eax
        movl    $gen_x86_64_shrd(int), %edx
        cmove   %rdx, %rax
        xorl    %edi, %edi
        jmp     *%rax


FWIW clang trunk also does not optimize this, yielding:
ix86_split_ashr(int):                   # @ix86_split_ashr(int)
        testl   %edi, %edi
        movl    $gen_x86_64_shrd(int), %eax
        movl    $ok(int), %ecx
        cmoveq  %rax, %rcx
        xorl    %edi, %edi
        jmpq    *%rcx                   # TAILCALL
ok(int):                                # @ok(int)
        movl    $1, %eax
        retq
gen_x86_64_shrd(int):                  # @gen_x86_64_shrd(int)
        xorl    %eax, %eax
        retq


ICC 19.0.1 _does_ optimize, resulting in:
ix86_split_ashr(int):
        ret                                                     #24.1
_INTERNALf242c0c5::gen_x86_64_shrd(int):
        xorl      %eax, %eax                                    #6.10
        ret                                                     #6.10
_INTERNALf242c0c5::ok(int):
        movl      $1, %eax                                      #12.10
        ret                                                     #12.10