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From: "mikulas at artax dot karlin.mff.cuni.cz" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug middle-end/114824] New: gcc doesn't respect
 __attribute__((zero_call_used_regs("used"))) when performing a tail call
 optimization
Date: Tue, 23 Apr 2024 10:24:19 +0000
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X-Bugzilla-Version: 14.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D114824

            Bug ID: 114824
           Summary: gcc doesn't respect
                    __attribute__((zero_call_used_regs("used"))) when
                    performing a tail call optimization
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: mikulas at artax dot karlin.mff.cuni.cz
  Target Milestone: ---

If a function has the attribute __attribute__((zero_call_used_regs("used")))
and the function ends with a sibling call, the used registers are not zeroe=
d.

See this function:
#include <stdio.h>
__attribute__((zero_call_used_regs("used"))) void f(void)
{
        __asm__ volatile("" ::: "x0", "x1", "x2", "x3", "x4", "x5", "x6",
"x7");
        printf("Hello World!\n");
}
Compile it with "gcc -S -o - -O2 call.c -Wall -Wextra" - the result is:
f:
        adrp    x0, .LC0
        add     x0, x0, :lo12:.LC0
        b       puts

If we disable the tail call optimization, the registers x0 - x7 are cleared
properly:
gcc -S -o - -O2 call.c -Wall -Wextra -fno-optimize-sibling-calls
f:
        stp     x29, x30, [sp, -16]!
        mov     x29, sp
        adrp    x0, .LC0
        add     x0, x0, :lo12:.LC0
        bl      puts
        ldp     x29, x30, [sp], 16
        mov     x0, 0
        mov     x1, 0
        mov     x2, 0
        mov     x3, 0
        mov     x4, 0
        mov     x5, 0
        mov     x6, 0
        mov     x7, 0
        mov     x16, 0
        mov     x17, 0
        ret

I tested it on arm64 and x86-64 and both of them suffer from this problem.=