[Bug gas/31685] New: Issues when casting _Float16 to __bf16

[mat.mcroci at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=31685

            Bug ID: 31685
           Summary: Issues when casting _Float16 to __bf16
           Product: binutils
           Version: 2.42
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: gas
          Assignee: unassigned at sourceware dot org
          Reporter: mat.mcroci at gmail dot com
  Target Milestone: ---

When compiling the following code (assuming it's in a file called test.cpp)

#include <stdlib.h>
#include <cstdint>
#include <array>
#include <iostream>

#define SIZE 8

typedef _Float16 T;
//typedef volatile float T;

void fp16tobf16(_Float16 * f) {
        __bf16 * b = reinterpret_cast<__bf16*>(f);
        for(int i=0; i<SIZE; i++){
            T temp = f[i];
            b[i] = (__bf16) temp;
        }
}

with:

g++ -c -S -march=sapphirerapids -O2 -std=c++23 -o test.s test.cpp

using gcc 13.2.0 compiled with binutils 2.42 using spack on a Sapphire Rapids
machine, the assembler does not do any casting and simply returns. You can also
check this behaviour on godbolt.

When using -O1 or -O0, the assembler generates the command __extendhfbf2 which
I assume correctly does the casting. However, when compiling the following very
similar code (assuming it's in a file called test2.cpp)

#include <stdlib.h>
#include <cstdint>
#include <array>
#include <iostream>

#define SIZE 8

typedef _Float16 T;
//typedef volatile float T;

void fp16tobf16(_Float16 * f) {
        __bf16 * b = reinterpret_cast<__bf16*>(f);
        for(int i=0; i<SIZE; i++){
            T temp = f[i];
            b[i] = (__bf16) temp;
        }
}

int main(){

    std::array<_Float16, SIZE> a{};
    std::fill(a.begin(), a.end(), (_Float16) 1.7653432432424324);
    fp16tobf16(a.data());

    __bf16 * b = reinterpret_cast<__bf16*>(a.data());

    std::cout << "\n";
    for(int i=0; i<SIZE; i++)
        std::cout << ((double) b[i]) << "\n";

    return 0;
}

with:

g++ -march=sapphirerapids -O0 -std=c++23 -o test2.run test2.cpp

The binutils ld triggers the error:

binutils-2.42-xfrnc4ulsgaca6jysot5cb7reykux4t6/bin/ld: /tmp/ccJJ5ihK.o: in
function `fp16tobf16(_Float16*)':
main_fp16_to_bf16.cpp:15:(.text+0x39): undefined reference to `__extendhfbf2'

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