[X86][GlobalIsel] In X87, no-sse2, returning floating point halts for SDAG, GISEL behavior differs.

[mahesh-attarde]

https://godbolt.org/z/9Ejz61Goh

For code with mattr=+87x,-sse2,-sse1

define float @test_float_abs(ptr %argptr)   {
     %arg = load  float, float* %argptr
     %abs = tail call float @llvm.fabs.f32(float %arg)
     ret float %abs
 }

SDAG

error: <unknown>:0:0: in function test_float_abs float (ptr): SSE register return with SSE disabled

Compiler returned: 1

GISEL

test_float_abs:                         # @test_float_abs
        mov     eax, 2147483647
        and     eax, dword ptr [rdi]
        movd    xmm0, eax
        ret

GISEL lowerreturn consults RETCC and adds xmm0 (Calling Convention) where as DAG does not do that.

[llvmbot]

@llvm/issue-subscribers-backend-x86

Author: Mahesh-Attarde (mahesh-attarde)

https://godbolt.org/z/9Ejz61Goh

For code with mattr=+87x,-sse2,-sse1

define float @<!-- -->test_float_abs(ptr %argptr)   {
     %arg = load  float, float* %argptr
     %abs = tail call float @<!-- -->llvm.fabs.f32(float %arg)
     ret float %abs
 }

SDAG

error: <unknown>:0:0: in function test_float_abs float (ptr): SSE register return with SSE disabled

Compiler returned: 1

GISEL

test_float_abs:                         # @<!-- -->test_float_abs
        mov     eax, 2147483647
        and     eax, dword ptr [rdi]
        movd    xmm0, eax
        ret

GISEL lowerreturn consults RETCC and adds xmm0 (Calling Convention) where as DAG does not do that.

[e-kud]

Worth to mention that

define x86_fp80 @test_float_abs(ptr %argptr)   {
     %arg = load  x86_fp80, x86_fp80* %argptr
     %abs = tail call x86_fp80 @llvm.fabs.f80(x86_fp80 %arg)
     ret x86_fp80 %abs
 }

Compiles fine with both selectors. So it is about floats and doubles returned using x87.