https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93985
Bug ID: 93985
Summary: Sub-optimal assembly for %st(0) constant loading with
SSE enabled (x86_64)
Product: gcc
Version: 9.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: sagebar at web dot de
Target Milestone: ---
Created attachment 47939
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=47939&action=edit
Inefficient code generation, similar working cases & work-around
With gcc for x86_64, where SSE is enabled by default, some situations exist
where legacy (fpu) math instructions (and their constraints) still have to be
used.
x86 offets a hand full of instructions to load specific floating point
constants more efficiently (including `1.0`).
As such, it would stand to reason to implement a function `atan()` as:
```c
double atan(double x) {
double result;
__asm__("fpatan"
: "=t" (result)
: "0" (1.0)
, "u" (x)
: "st(1)");
return result;
}
```
This code works perfectly on i386, where it compiles to:
```asm
fldl 4(%esp) # push(x)
fld1 # push(1.0)
fpatan
ret
```
However, on x86_64 it is compiled as:
```asm
movsd .LC0(%rip), %xmm1
movsd %xmm1, -8(%rsp)
fldl -8(%rsp) # push(1.0)
movsd %xmm0, -8(%rsp)
fldl -8(%rsp) # push(x)
fxch %st(1) # { x, 1.0 } -> { 1.0, x }
fpatan
fstpl -8(%rsp)
movsd -8(%rsp), %xmm0
ret
...
.LC0:
.long 0 # SSE constant: 1.0
.long 1072693248 # ...
```
When the optimal code would look like:
```asm
movsd %xmm0, -8(%rsp)
fldl -8(%rsp) # push(x)
fld1 # push(1.0)
fpatan
fstpl -8(%rsp)
movsd -8(%rsp), %xmm0
ret
```
Still though, it appears that GCC _is_ aware of the fld1 instruction for
encoding inline assembly operands, even when SSE is enabled (s.a.
`atan_reverse()` within the attached file).
So it would stand to reason that this is either a problem with how GCC weights
different encoding schemes, or a problem with how GCC decides if certain
encoding schemes are even possible at all (which seems quite likely, especially
considering that the x86_64 version contains an fxch-instruction which also
wouldn't be necessary if GCC had encoded the `1.0` _after_ pushing `x`
(ignoring the fact that `1.0` can be pushed using `fld1`))
NOTE: The attached file should be compiled as `gcc -O3 -S attached-file.c`
