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* [Bug target/106327] New: side-effect-free _x variance not optimized to unpredicated instruction
@ 2022-07-16 20:11 yyc1992 at gmail dot com
2022-08-31 11:38 ` [Bug target/106327] " rsandifo at gcc dot gnu.org
0 siblings, 1 reply; 2+ messages in thread
From: yyc1992 at gmail dot com @ 2022-07-16 20:11 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106327
Bug ID: 106327
Summary: side-effect-free _x variance not optimized to
unpredicated instruction
Product: gcc
Version: 12.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: yyc1992 at gmail dot com
Target Milestone: ---
Related to https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106326 .
According to the Arm C Language Extension for SVE, when the _x predicate is
used,
> The compiler can then pick whichever form of instruction seems to give the best code. This includes using unpredicated instructions, where available and suitable
Because of this, I'm expecting the following to be optimized to a single add
instruction, as if a `svptrue_b64()` predicate is used.
```
svfloat64_t add(svfloat64_t a, svfloat64_t b)
{
auto und_ok = svcmpge(svptrue_b64(), a, b);
return svadd_x(und_ok, a, b);
}
```
However, gcc compiles this as _m and generates
```
ptrue p0.b, all
fcmge p0.d, p0/z, z0.d, z1.d
fadd z0.d, p0/m, z0.d, z1.d
```
In general, is there any reason not to treat an `add_x` (also other
side-effect-free functions) with an unknown predicate as unpredicated one?
^ permalink raw reply [flat|nested] 2+ messages in thread
* [Bug target/106327] side-effect-free _x variance not optimized to unpredicated instruction
2022-07-16 20:11 [Bug target/106327] New: side-effect-free _x variance not optimized to unpredicated instruction yyc1992 at gmail dot com
@ 2022-08-31 11:38 ` rsandifo at gcc dot gnu.org
0 siblings, 0 replies; 2+ messages in thread
From: rsandifo at gcc dot gnu.org @ 2022-08-31 11:38 UTC (permalink / raw)
To: gcc-bugs
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106327
rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Ever confirmed|0 |1
Status|UNCONFIRMED |WAITING
CC| |rsandifo at gcc dot gnu.org
Last reconfirmed| |2022-08-31
--- Comment #1 from rsandifo at gcc dot gnu.org <rsandifo at gcc dot gnu.org> ---
This is because performing the addition on the inactive lanes
could trigger an IEEE exception. The code is optimised to an
unpredicated FADD with -ffast-math.
^ permalink raw reply [flat|nested] 2+ messages in thread
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2022-07-16 20:11 [Bug target/106327] New: side-effect-free _x variance not optimized to unpredicated instruction yyc1992 at gmail dot com
2022-08-31 11:38 ` [Bug target/106327] " rsandifo at gcc dot gnu.org
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