https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79665
Bug ID: 79665
Summary: gcc's signed (x*x)/200 is slower than clang's
Product: gcc
Version: 7.0
URL: https://github.com/rust-lang/rust/issues/39446#issueco
mment-280798073
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: jistone at redhat dot com
Target Milestone: ---
Created attachment 40809
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=40809&action=edit
C source from rust-lang/rust#39446
The linked Rust issue is initially comparing rustc and clang, but I found that
GCC is also generating worse code than clang. Profiling pointed me to these:
x_x = (x * x) / 200;
y_y = (y * y) / 200;
Clang optimizes these to just 4 instructions each, like:
movl %edx, %ebp
imull %ebp, %ebp
imulq $1374389535, %rbp, %rbp # imm = 0x51EB851F
shrq $38, %rbp
But clang -fwrapv generates 8 instructions each, same as rustc, and it turns
out GCC uses a similar longer sequence too regardless of -fwrapv. GCC 6.3
gives:
movl %edi, %r13d
imull %edi, %r13d
movl %r13d, %eax
sarl $31, %r13d
imull %ebx
sarl $6, %edx
movl %edx, %ecx
subl %r13d, %ecx
And GCC 7 is the same, although it interleaves the two x and y lines.
Here they are on Compiler Explorer: https://godbolt.org/g/Fy8Fiy
I believe what's happening is that clang identifies that signed (x*x) is always
positive, thanks to overflow UB, and then (x*x)/200 is essentially unsigned
division, which requires less fixup than signed constant division.
