[LLVMbugs] [Bug 12251] New: clang produces unnecessary 'and' after loading a bool

[bugzill...@llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=12251

Bug #: 12251
Summary: clang produces unnecessary 'and' after loading a bool
Product: clang
Version: unspecified
Platform: PC
OS/Version: All
Status: NEW
Severity: enhancement
Priority: P
Component: LLVM Codegen
AssignedTo: unassigne...@nondot.org
ReportedBy: rafael.e...@gmail.com
CC: llvm...@cs.uiuc.edu
Classification: Unclassified

The X86-64 abi says:

Booleans, when stored in a memory object, are stored as single byte objects the
value of which is always 0 (false) or 1 (true).

For the function

bool foo(bool *x) {
return *x;
}

clang produces this IL (after optimizations):

define zeroext i1 @_Z3fooPb(i8* nocapture %x) nounwind uwtable readonly ssp {
entry:
%0 = load i8* %x, align 1, !tbaa !0
%1 = and i8 %0, 1
%tobool = icmp ne i8 %1, 0
ret i1 %tobool
}

which ends up producing:

movb (%rdi), %al
andb $1, %al
ret

Producing instead

define zeroext i1 @_Z3fooPb2(i8* nocapture %x) nounwind uwtable readonly ssp {
entry:
%x2 = bitcast i8* %x to i1*
%tmp0 = load i1* %x2, align 1
ret i1 %tmp0
}

would result in

movb (%rdi), %al
ret

Apple's gcc 4.2 produces

movzbl (%rdi), %eax
ret

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[bugzill...@llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=12251

Rafael Ávila de Espíndola <rafael.e...@gmail.com> changed:

What |Removed |Added
----------------------------------------------------------------------------
Status|ASSIGNED |RESOLVED
Resolution| |FIXED

--- Comment #12 from Rafael Ávila de Espíndola <rafael.e...@gmail.com> 2012-03-26 10:32:19 CDT ---
The 'and' is gone, but we are still not as efficient as gcc. I opened pr12360
to track the missing improvements.