problems with soft-float

[llvmbot]

Bugzilla Link	45004
Version	7.0
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@topperc,@DougGregor,@zygoloid

Extended Description

When I compile this program:

#include <stdio.h>
int main(int argc, char*argv[])
{
    double x = 2.0;
    double y = 3.0;
    printf("%f\n", x * y);
    return 0;
}

with "clang -msoft-float -mno-sse -O0 test.c", it prints 0 instead of 6.

Output of clang --version:

clang --version clang version 7.0.1-8 (tags/RELEASE_701/final) Target: x86_64-pc-linux-gnu Thread model: posix InstalledDir: /usr/bin

I'm trying to use soft-float without any FPU or XMM registers, as it is e.g. possible on ARM.

[llvmbot]

You are right, looks like this is the problem. The calling convention for AMD64 says so, too:

https://en.wikipedia.org/wiki/X86_calling_conventions#System_V_AMD64_ABI

I tested it with this program:

#include <stdio.h>
#include <stdint.h>
#include <inttypes.h>

union N {
    double d;
    uint64_t u;
};

int main(int argc, char*argv[])
{
    union N x;
    union N y;
    union N z;
    x.d = 2.0;
    y.d = 3.0;
    z.d = x.d * y.d;
    printf("%"PRId64"\n", z.u);
    printf("%f\n", z.d);
    return 0;
}

and it prints this:

4618441417868443648
0.000000

I verified that the first number is the representation for the double float value 6, so it does at least the calculation.

I think this is a bug in CLANG, because when compiling it without the "-mno-sse", it places it in the XMM0 value, so CLANG knows that it has to place it in this register. If I specify that not XMM registers should be used, then it should abort with an error, because it can't place the value in this register anymore.

But I also noticed that it still generates fmul. Is this right? I thought with "-msoft-float" it shouldn't use the FPU instructions. This is the full main function, disassembled with "objdump -d":

0000000000401130 <main>:
  401130:       55                      push   %rbp
  401131:       48 89 e5                mov    %rsp,%rbp
  401134:       48 83 ec 30             sub    $0x30,%rsp
  401138:       c7 45 fc 00 00 00 00    movl   $0x0,-0x4(%rbp)
  40113f:       89 7d f8                mov    %edi,-0x8(%rbp)
  401142:       48 89 75 f0             mov    %rsi,-0x10(%rbp)
  401146:       48 be 00 00 00 00 00    movabs $0x4000000000000000,%rsi
  40114d:       00 00 40 
  401150:       48 89 75 e8             mov    %rsi,-0x18(%rbp)
  401154:       48 be 00 00 00 00 00    movabs $0x4008000000000000,%rsi
  40115b:       00 08 40 
  40115e:       48 89 75 e0             mov    %rsi,-0x20(%rbp)
  401162:       dd 45 e8                fldl   -0x18(%rbp)
  401165:       dd 45 e0                fldl   -0x20(%rbp)
  401168:       de c9                   fmulp  %st,%st(1)
  40116a:       48 89 e6                mov    %rsp,%rsi
  40116d:       dd 1e                   fstpl  (%rsi)
  40116f:       bf 04 20 40 00          mov    $0x402004,%edi
  401174:       31 c0                   xor    %eax,%eax
  401176:       88 c1                   mov    %al,%cl
  401178:       88 c8                   mov    %cl,%al
  40117a:       e8 b1 fe ff ff          callq  401030 <printf@plt>
  40117f:       31 d2                   xor    %edx,%edx
  401181:       89 45 dc                mov    %eax,-0x24(%rbp)
  401184:       89 d0                   mov    %edx,%eax
  401186:       48 83 c4 30             add    $0x30,%rsp
  40118a:       5d                      pop    %rbp
  40118b:       c3                      retq   
  40118c:       0f 1f 40 00             nopl   0x0(%rax)

[topperc]

I think the problem is that printf doesn’t know about the soft float switch since it’s in a precompiled library. It expects to receive floating point data in an xmm register. But the soft float setting prevents the compiler from generating any code that uses an xmm register.

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: None (llvmbot)

| | | | --- | --- | | Bugzilla Link | [45004](https://llvm.org/bz45004) | | Version | 7.0 | | OS | Linux | | Reporter | LLVM Bugzilla Contributor | | CC | @topperc,@DougGregor,@zygoloid | ## Extended Description When I compile this program: #include <stdio.h> int main(int argc, char*argv[]) { double x = 2.0; double y = 3.0; printf("%f\n", x * y); return 0; } with "clang -msoft-float -mno-sse -O0 test.c", it prints 0 instead of 6. Output of clang --version: clang --version clang version 7.0.1-8 (tags/RELEASE_701/final) Target: x86_64-pc-linux-gnu Thread model: posix InstalledDir: /usr/bin I'm trying to use soft-float without any FPU or XMM registers, as it is e.g. possible on ARM.