[Bug regression/21463] New: min/max and references

["tbptbp at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

I don't quite know how to characterize this one, so i'll let it up to those in
the know to fix the summary/description.
Primo, AFAIK, gcc has always struggled to get this right but 4.1 is setting a
new record; so i'll qualify that as a regression vs 3.3/3.4.
Secundo, i haven't found any related bugreports but one about PPC.
So here we go.

Consider this testcase:
#include <algorithm>

template<class T> static inline T max(const T a, const T b) { return a<b ? b : a; }
template<class T> static inline T min(const T a, const T b) { return a<b ? a : b; }

template<class T> static inline const T &ref_max(const T &a, const T &b) {
return a<b ? b : a; }
template<class T> static inline const T &ref_min(const T &a, const T &b) {
return a<b ? a : b; }

template<class T> struct foo_t {
	T a0, a1;
	T bar(const T b, const T c) {
		return max(min(a0, c), min(max(a1, c), b));
	}
	T bar_ref(const T b, const T c) {
		return ref_max(ref_min(a0, c), ref_min(ref_max(a1, c), b));
	}
	T bar_stl(const T b, const T c) {
		return std::max(std::min(a0, c), std::min(std::max(a1, c), b));
	}
};
template struct foo_t<int>;
template struct foo_t<float>;

int main() { return 0; }

With g++-4120050501 -O3 i get such creative & entertaining code as:
0000000000400610 <foo_t<float>::bar_ref(float, float)>:
  400610:       ucomiss 0x4(%rdi),%xmm1
  400614:       lea    0x4(%rdi),%rax
  400618:       lea    0xfffffffffffffff8(%rsp),%rdx
  40061d:       movss  %xmm0,0xfffffffffffffffc(%rsp)
  400623:       movss  %xmm1,0xfffffffffffffff8(%rsp)
  400629:       movaps %xmm1,%xmm2
  40062c:       cmova  %rdx,%rax
  400630:       movss  (%rax),%xmm1
  400634:       ucomiss %xmm1,%xmm0
  400637:       ja     400641 <foo_t<float>::bar_ref(float, float)+0x31>
  400639:       lea    0xfffffffffffffffc(%rsp),%rax
  40063e:       movaps %xmm0,%xmm1
  400641:       ucomiss (%rdi),%xmm2
  400644:       cmova  %rdi,%rdx
  400648:       movss  (%rdx),%xmm0
  40064c:       ucomiss %xmm0,%xmm1
  40064f:       jbe    400655 <foo_t<float>::bar_ref(float, float)+0x45>
  400651:       movss  (%rax),%xmm0
  400655:       repz retq

Compare that to what g++-3.4.4-20050314 gives (g++ 3.3.6 is similar):
0000000000400610 <foo_t<float>::bar_ref(float, float)>:
  400610:       ucomiss (%rdi),%xmm1
  400613:       lea    0xfffffffffffffffc(%rsp),%rsi
  400618:       mov    %rdi,%rcx
  40061b:       lea    0x4(%rdi),%rdx
  40061f:       movss  %xmm0,0xfffffffffffffff8(%rsp)
  400625:       lea    0xfffffffffffffff8(%rsp),%rax
  40062a:       movss  %xmm1,0xfffffffffffffffc(%rsp)
  400630:       cmovbe %rsi,%rcx
  400634:       ucomiss 0x4(%rdi),%xmm1
  400638:       cmova  %rsi,%rdx
  40063c:       ucomiss (%rdx),%xmm0
  40063f:       cmova  %rdx,%rax
  400643:       movss  (%rax),%xmm0
  400647:       ucomiss (%rcx),%xmm0
  40064a:       cmovbe %rcx,%rax
  40064e:       movss  (%rax),%xmm0
  400652:       retq
Certainly not optimal, and in fact quite ugly, but at least there's no branch.

Happens as soon as references are used and enough min/max are piled up, and that
means that unless you use your own min/max instead of the STL version, you're
doomed.

-- 
           Summary: min/max and references
           Product: gcc
           Version: 4.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: regression
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: tbptbp at gmail dot com
                CC: gcc-bugs at gcc dot gnu dot org
  GCC host triplet: x86*


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21463