https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70171
Bug ID: 70171
Summary: Poor code generated when return struct using ternary
operator
Product: gcc
Version: 5.3.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: sethml at ofb dot net
Target Milestone: ---
Created attachment 37926
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=37926&action=edit
Example C++ code.
Consider the following code:
int int_ternary(int a, int b, bool select) { return select ? a : b; }
int int_if(int a, int b, bool select) { if (select) return a; else return b; }
struct S { int i; };
S struct_ternary(S a, S b, bool select) { return select ? a : b; }
S struct_if(S a, S b, bool select) { if (select) return a; else return b; }
G++ 5.3.0 -O2 generates identical code for int_ternary(), int_if(), and
struct_if() - for x86-64:
testb %dl, %dl
movl %esi, %eax
cmovne %edi, %eax
ret
However, for struct_ternary() it generates terrible code which stores a and b
into memory locations and then loads one by address:
struct_ternary(S, S, bool):
leaq -40(%rsp), %rcx
testb %dl, %dl
movl %edi, -24(%rsp)
leaq -24(%rsp), %rax
movl %esi, -40(%rsp)
cmove %rcx, %rax
movl (%rax), %eax
ret
Compile command:
% g++-5 -O2 -S ternary_reference_pessimization.cc
% g++-5 --version
g++-5 (Homebrew gcc 5.3.0) 5.3.0
This can have a major performance impact when using wrappers around built-in
types. For example, I discovered it experimenting with John McFarlane's
fixed_point library, which is a candidate for C++ standardization. In
particular, calling std::max() on two fixed_point<uint32_t,...> values results
in the kind of terrible code demonstrated above.
https://github.com/johnmcfarlane/fixed_point/blob/master/doc/p0037r1.md
As wrappers around basic types become more common in C++, the performance
impact of this problem will get worse.
