[LLVMbugs] [Bug 3739] New: x86-64 generates incorrect asm in function prolog/ epilog trying to save XMM registers

[bugzilla-daemon at cs.uiuc.edu]

http://llvm.org/bugs/show_bug.cgi?id=3739

           Summary: x86-64 generates incorrect asm in function prolog/epilog
                    trying to save XMM registers
           Product: libraries
           Version: 2.5
          Platform: PC
        OS/Version: Windows XP
            Status: NEW
          Severity: normal
          Priority: P2
         Component: Backend: X86
        AssignedTo: unassignedbugs at nondot.org
        ReportedBy: craig at ni.com
                CC: evan.cheng at apple.com, asl at math.spbu.ru, dalej at apple.com,
                    llvmbugs at cs.uiuc.edu, nicolas at capens.net
        Depends on: 2801


On Win64, registers XMM6 - XMM15 are considered non-volatile and should be
callee-saved.
The bug  this bug is a clone of (#2801) attempted to fix this, but the patch
suggested as the fix  was only partially submitted; it added the XMM registers
to X86RegisterInfo::getCalleeSavesRegs(), but that's all.   

The X86 function prolog/epilog code does not know how to save/restore XMM
registers and tries to emit a PUSH XMMn instruction, which is invalid; it emits
bad assembly.  (When jitting, it encodes a push of a GP register instead.)

For example:

target triple = "x86_64-pc-windows"

declare extern_weak i32 @foo()

define i32 @func() {
entry:
        %r = call i32 @foo() nounwind
        ret i32 %r
}

% llvm-as < test.ll | llc -x86-asm-syntax=att
(I can't stand Intel asm syntax, so even though this is a Win64 target I try to
lessen the pain)

_text   segment 'DATA'
        align   16
        .globl  _func
_func:
$label1:
        pushq   %xmm15
        pushq   %xmm14
        pushq   %xmm13
        pushq   %xmm12
        pushq   %xmm11
        pushq   %xmm10
        pushq   %xmm9
        pushq   %xmm8
        pushq   %xmm7
        pushq   %xmm6
        pushq   %rsi
        pushq   %rdi
        subq    $88, %rsp
        call    _foo
        addq    $88, %rsp
        popq    %rdi
        popq    %rsi
        popq    %xmm6
        popq    %xmm7
        popq    %xmm8
        popq    %xmm9
        popq    %xmm10
        popq    %xmm11
        popq    %xmm12
        popq    %xmm13
        popq    %xmm14
        popq    %xmm15
        ret

----
Note also that not only does the prolog/epilog emit incorrect instructions for
dealing with XMM*,
it also shouldn't be bothering to save/restore the registers in the first place
since they are not used in the function.  (In
PEI::calculateCalleeSavedRegisters(), the Fn.getRegInfo().isPhysRegUsed(Reg)
call always returns true for all the XMM registers if the Function being
emitted has any call instructions.)

The part of the patch attached to #2801 which was not applied did attempt to
generate direct writes/reads to the stack for XMM registers instead of
push/pop, but still doesn't ensure alignment correctly.


+++ This bug was initially created as a clone of Bug #2801 +++

The x86-64 ABI specifies that XMM6 to XMM15 are non-volatile, and should be
preserved by the callee as needed. It appears that LLVM currently doesn't do
this, causing unpredictable behavior with floating-point calculations.


-- 
Configure bugmail: http://llvm.org/bugs/userprefs.cgi?tab=email
------- You are receiving this mail because: -------
You are on the CC list for the bug.