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digitalmars.D.learn - Using shorthand *= leads to unexpected result?

reply Michael <michael toohuman.io> writes:
It may be that I'm doing something wrong here, but after updating 
DMD to the latest version, my simulations started producing some 
very odd results and I think I've pinpointed it to a sign 
inversion that I was making. Here is some code from dpaste to 
demonstrate the behaviour I get vs. the behaviour I expected:
https://dpaste.dzfl.pl/9bd7aea75fb2

Any ideas? Am I getting something wrong or is this some sort of 
regression with the latest DMD?
May 15 2016
next sibling parent reply Saurabh Das <saurabh.das gmail.com> writes:
On Sunday, 15 May 2016 at 13:01:45 UTC, Michael wrote:
 It may be that I'm doing something wrong here, but after 
 updating DMD to the latest version, my simulations started 
 producing some very odd results and I think I've pinpointed it 
 to a sign inversion that I was making. Here is some code from 
 dpaste to demonstrate the behaviour I get vs. the behaviour I 
 expected:
 https://dpaste.dzfl.pl/9bd7aea75fb2

 Any ideas? Am I getting something wrong or is this some sort of 
 regression with the latest DMD?
Strangely, if I substitute the variable type 'auto' for 'float' or 'real', it gives the expected output of -1. Variable type 'double' gives the incorrect value of 1. This is strange behaviour. Perhaps someone more knowledgeable can shed light. If it is a regression, it is a very serious regression :( ~ sd
May 15 2016
parent reply Michael <michael toohuman.io> writes:
On Sunday, 15 May 2016 at 13:12:44 UTC, Saurabh Das wrote:
 On Sunday, 15 May 2016 at 13:01:45 UTC, Michael wrote:
 It may be that I'm doing something wrong here, but after 
 updating DMD to the latest version, my simulations started 
 producing some very odd results and I think I've pinpointed it 
 to a sign inversion that I was making. Here is some code from 
 dpaste to demonstrate the behaviour I get vs. the behaviour I 
 expected:
 https://dpaste.dzfl.pl/9bd7aea75fb2

 Any ideas? Am I getting something wrong or is this some sort 
 of regression with the latest DMD?
Strangely, if I substitute the variable type 'auto' for 'float' or 'real', it gives the expected output of -1. Variable type 'double' gives the incorrect value of 1. This is strange behaviour. Perhaps someone more knowledgeable can shed light. If it is a regression, it is a very serious regression :( ~ sd
Well I'm pretty sure the code was working just fine earlier in the week at the office, but running the code at home with the newest version of DMD started producing these odd results.
May 15 2016
parent Saurabh Das <saurabh.das gmail.com> writes:
On Sunday, 15 May 2016 at 13:25:42 UTC, Michael wrote:
 Well I'm pretty sure the code was working just fine earlier in 
 the week at the office, but running the code at home with the 
 newest version of DMD started producing these odd results.
Typing this function into asm.dlang.org shows a minor difference in the assembly generated: auto f1(double d1) { d1 *= -1.0; return d1; } With DMD 2.070.2: pure nothrow nogc safe double example.f1(double): push %rbp mov %rsp,%rbp sub $0x10,%rsp movsd %xmm0,-0x8(%rbp) xorb $0x80,-0x1(%rbp) rex.W movsd -0x8(%rbp),%xmm0 leaveq retq nopl 0x0(%rax) With DMD 2.071.0: pure nothrow nogc safe double example.f1(double): push %rbp mov %rsp,%rbp sub $0x10,%rsp movsd %xmm0,-0x8(%rbp) xorb $0x80,-0x8(%rbp) rex.W movsd -0x8(%rbp),%xmm0 leaveq retq nopl 0x0(%rax) The xorb line is different and I conjecture that it is causing the bug. I have never used floating-point assembly instructions, so I cannot understand what the logic is here. I'll read up and try to interpret to confirm. Hope this helps.
May 15 2016
prev sibling parent reply Marc =?UTF-8?B?U2Now7x0eg==?= <schuetzm gmx.net> writes:
On Sunday, 15 May 2016 at 13:01:45 UTC, Michael wrote:
 It may be that I'm doing something wrong here, but after 
 updating DMD to the latest version, my simulations started 
 producing some very odd results and I think I've pinpointed it 
 to a sign inversion that I was making. Here is some code from 
 dpaste to demonstrate the behaviour I get vs. the behaviour I 
 expected:
 https://dpaste.dzfl.pl/9bd7aea75fb2

 Any ideas? Am I getting something wrong or is this some sort of 
 regression with the latest DMD?
It's a regression introduced here: https://github.com/dlang/dmd/pull/5534 I've filed a bug report: https://issues.dlang.org/show_bug.cgi?id=16027
May 15 2016
parent Michael <michael toohuman.io> writes:
On Sunday, 15 May 2016 at 14:12:47 UTC, Marc Sch├╝tz wrote:
 On Sunday, 15 May 2016 at 13:01:45 UTC, Michael wrote:
 It may be that I'm doing something wrong here, but after 
 updating DMD to the latest version, my simulations started 
 producing some very odd results and I think I've pinpointed it 
 to a sign inversion that I was making. Here is some code from 
 dpaste to demonstrate the behaviour I get vs. the behaviour I 
 expected:
 https://dpaste.dzfl.pl/9bd7aea75fb2

 Any ideas? Am I getting something wrong or is this some sort 
 of regression with the latest DMD?
It's a regression introduced here: https://github.com/dlang/dmd/pull/5534 I've filed a bug report: https://issues.dlang.org/show_bug.cgi?id=16027
Thanks for filing the bug report, and good find with the regression.
May 15 2016