[Bug c/43639] Missed optimization with complex long double

[jsm28 at gcc dot gnu dot org]

--- Comment #4 from jsm28 at gcc dot gnu dot org  2010-05-22 18:18 ---
This is deliberate, use -fno-signed-zeros to get the optimization you
want with 4.5.


*** This bug has been marked as a duplicate of 24581 ***


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jsm28 at gcc dot gnu dot org changed:

   What|Removed |Added

 Status|UNCONFIRMED |RESOLVED
 Resolution||DUPLICATE


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43639

[Bug c/43639] Missed optimization with complex long double

[pinskia at gmail dot com]

--- Comment #1 from pinskia at gmail dot com  2010-04-04 06:28 ---
Subject: Re:   New: Missed optimization with complex long double



Sent from my iPhone

On Apr 3, 2010, at 11:21 PM, svfuerst at gmail dot com
gcc-bugzi...@gcc.gnu.org 
  wrote:

 gcc 4.4 compiles the following:

 _Complex long double foo(long double p1, long double p2)
 {
return p1 + (__extension__ 1.0iF) * p2;
 }

 gcc-4.4 -O3 tgcc.c -c -o tgcc.o

 into

   0x +0: fldt   0x8(%rsp)
   0x0004 +4: fldt   0x18(%rsp)
   0x0008 +8: fxch   %st(1)
   0x000a +10:retq

 This is ok, except for the useless fxch instruction.  However, gcc  
 4.5 compiles
 the same code into:
 gcc-4.5 -O3 tgcc.c -c -o tgcc.o

   0x +0: fldt   0x18(%rsp)
   0x0004 +4: fld%st(0)
   0x0006 +6: fmuls  0x0(%rip)# 0xc foo+12
   0x000c +12:fldt   0x8(%rsp)
   0x0010 +16:faddp  %st,%st(1)
   0x0012 +18:retq

 which is quite a bit worse.

Except it is needed for handling -0.0 correctly.



 -- 
   Summary: Missed optimization with complex long double
   Product: gcc
   Version: unknown
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: c
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: svfuerst at gmail dot com
 GCC build triplet: x86_64-linux
  GCC host triplet: x86_64-linux
 GCC target triplet: x86_64-linux


 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43639



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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43639

[Bug c/43639] Missed optimization with complex long double

[svfuerst at gmail dot com]

--- Comment #2 from svfuerst at gmail dot com  2010-04-04 16:51 ---
Paragraph 2 in G.5.1 defines multiplication of a real floating point type by an
imaginary floating point type to be an imaginary type, not a complex type.

In addition, the Rationale for Annex G states that

It allow writing imaginary and complex expressions in common mathematical
style, for example x + I*y.  Note that the multiplication here affects
translated code, but does not necessitate an actual floating-point multiply,
nor does the addition necessitate a floating point add.

Which strongly suggests that the naive code generation which ignores the signed
zero issue is the correct one. (Note that the rationale also lists the case
where y=INFINITY as causing problems with signaling NaNs if I is _Complex_I.)


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43639

[Bug c/43639] Missed optimization with complex long double

[joseph at codesourcery dot com]

--- Comment #3 from joseph at codesourcery dot com  2010-04-04 19:12 ---
Subject: Re:  Missed optimization with complex long double

On Sun, 4 Apr 2010, svfuerst at gmail dot com wrote:

 Paragraph 2 in G.5.1 defines multiplication of a real floating point 
 type by an imaginary floating point type to be an imaginary type, not a 
 complex type.

GCC does not support imaginary types, so this is irrelevant.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43639