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From: "stefan.sakalik at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug rtl-optimization/111744] New: Missed optimization when casting rdtsc into uint32_t and computing difference
Date: Mon, 09 Oct 2023 19:17:22 +0000	[thread overview]
Message-ID: <bug-111744-4@http.gcc.gnu.org/bugzilla/> (raw)

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111744

            Bug ID: 111744
           Summary: Missed optimization when casting rdtsc into uint32_t
                    and computing difference
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stefan.sakalik at gmail dot com
  Target Milestone: ---

This is similar to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92180 where

this code: https://godbolt.org/z/7W9nqTsjE

#include <bits/stdc++.h>
#include <x86intrin.h>

uint32_t rdtsc32() { return static_cast<uint32_t>(__rdtsc()); }

uint64_t rdtsc_delta(uint64_t x) {
    return rdtsc32() - rdtsc32();
}

Produces 

rdtsc_delta(unsigned long):
        rdtsc
        mov     rcx, rax
        sal     rdx, 32
        or      rcx, rdx
        rdtsc
        sub     ecx, eax
        mov     rax, rcx
        ret

as opposed to clang version

rdtsc_delta(unsigned long):
        rdtsc
        mov     rcx, rax
        rdtsc
        sub     ecx, eax
        mov     rax, rcx
        ret

                 reply	other threads:[~2023-10-09 19:17 UTC|newest]

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