[llvm-dev] Question on using caller saved register around a library call.

[Venkataramanan Kumar via llvm-dev]

Is that the case, all XMM, YMM and ZMM are caller saved or scratch only ?
 As per the ABI, we don't have any Callee saved registers available?

regards,
Venkat.


On Thu, 18 Mar 2021 at 20:37, Venkataramanan Kumar <
venkataramanan.kumar.llvm at gmail.com> wrote:

> Hi,
>
> For the below test case, I see LLVM allocates an XMM1 register for holding
> the variable "t".  My understanding is XMM1 is a caller saved register and
> hence it is spilled to stack and restored back around the library call.
>
> Ref: https://godbolt.org/z/Peaczh
>
> --snip--
> #include <math.h>
> double arr[1000000];
> int n;
> double foo()
> {
>    double t = 0.0;
>    for(int i=0;i<100000;i++)
>      t += log(arr[i]);
>
>    return t;
> }
> --snip--
>
> Assembly Snip
> ---Snip--
> .LBB0_1:                                # =>This Inner Loop Header: Depth=
> 1
>         #DEBUG_VALUE: foo:t <- 0.000000e+00
>         #DEBUG_VALUE: i <- 0
>         *vmovupd %ymm1, (%rsp)                   # 32-byte Spill*
> .Ltmp1:
>         .loc    1 8 15 prologue_end             # ./example.cpp:8:15
>         vmovupd arr+800000(%rbx), %ymm0
>         .loc    1 8 11 is_stmt 0                # ./example.cpp:8:11
>         callq   __svml_log4 at PLT
> .Ltmp2:
>         .loc    1 0 11                          # ./example.cpp:0:11
>         *vmovupd (%rsp), %ymm1                   # 32-byte Reload*
>         .loc    1 8 8                           # ./example.cpp:8:8
>         vaddpd  %ymm1, %ymm0, %ymm1
>         .loc    1 7 26 is_stmt 1                # ./example.cpp:7:26
>         addq    $32, %rbx
>         jne     .LBB0_1
> ---Snip---
>
> Why is LLVM not choosing a callee saved register here?  It can then avoid
> pushing to the stack and restoring from the stack in the loop.
>
> regards,
> Venkat.
>
>
>
On Thu, 18 Mar 2021 at 20:37, Venkataramanan Kumar <
venkataramanan.kumar.llvm at gmail.com> wrote:

> Hi,
>
> For the below test case, I see LLVM allocates an XMM1 register for holding
> the variable "t".  My understanding is XMM1 is a caller saved register and
> hence it is spilled to stack and restored back around the library call.
>
> Ref: https://godbolt.org/z/Peaczh
>
> --snip--
> #include <math.h>
> double arr[1000000];
> int n;
> double foo()
> {
>    double t = 0.0;
>    for(int i=0;i<100000;i++)
>      t += log(arr[i]);
>
>    return t;
> }
> --snip--
>
> Assembly Snip
> ---Snip--
> .LBB0_1:                                # =>This Inner Loop Header: Depth=
> 1
>         #DEBUG_VALUE: foo:t <- 0.000000e+00
>         #DEBUG_VALUE: i <- 0
>         *vmovupd %ymm1, (%rsp)                   # 32-byte Spill*
> .Ltmp1:
>         .loc    1 8 15 prologue_end             # ./example.cpp:8:15
>         vmovupd arr+800000(%rbx), %ymm0
>         .loc    1 8 11 is_stmt 0                # ./example.cpp:8:11
>         callq   __svml_log4 at PLT
> .Ltmp2:
>         .loc    1 0 11                          # ./example.cpp:0:11
>         *vmovupd (%rsp), %ymm1                   # 32-byte Reload*
>         .loc    1 8 8                           # ./example.cpp:8:8
>         vaddpd  %ymm1, %ymm0, %ymm1
>         .loc    1 7 26 is_stmt 1                # ./example.cpp:7:26
>         addq    $32, %rbx
>         jne     .LBB0_1
> ---Snip---
>
> Why is LLVM not choosing a callee saved register here?  It can then avoid
> pushing to the stack and restoring from the stack in the loop.
>
> regards,
> Venkat.
>
>
>
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