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From: "zero at smallinteger dot com" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug target/100391] New: 128 bit arithmetic --- many unnecessary
 instructions when extracting smaller parts
Date: Mon, 03 May 2021 08:43:55 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100391

            Bug ID: 100391
           Summary: 128 bit arithmetic --- many unnecessary instructions
                    when extracting smaller parts
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: zero at smallinteger dot com
  Target Milestone: ---

Created attachment 50738
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D50738&action=3Dedit
Sample code

Consider the attached code, compiled with -O2.  The return value of both
functions is just the low 32 bits of num.  Whether the top 4 bits of kt were
zero, or became zero because of the shifts in the if statement, is irreleva=
nt.=20
So, this both functions should have resulted in something like

twostep(unsigned __int128):                            # @twostep(unsigned
__int128)
        mov     rax, rdi
        ret
onestep(unsigned __int128):                            # @onestep(unsigned
__int128)
        mov     rax, rdi
        ret


Instead, gcc added many unnecessary instructions to twostep() as shown belo=
w.

twostep(unsigned __int128):
        mov     rcx, rdi
        mov     rax, rdi
        shr     rcx, 60
        je      .L2
        movabs  rdx, 1152921504606846975
        and     rax, rdx
.L2:
        ret
onestep(unsigned __int128):
        mov     rax, rdi
        ret


This particular behavior was isolated while examining the output of gcc 9.3=
.0
on Ubuntu 20.04 LTS, then verified for the stated versions (and a few other=
s)
using Godbolt.

Incidentally, it might be worth checking whether movabs + and is indeed fas=
ter
than shl + shr, assuming doing so was necessary.  If too many movabs
instructions are generated for bit masking like this, it will run against t=
he
Intel optimization manual's recommendation not to include too many full size
literals in code.=