optimization gcc vs fpc
lazarus / 64-FPC development

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fcu · 2019-04-23 13:49

Hi
just looking how fpc optimize this function compared with how gcc does
gcc seems smarter, it reduce it to one instruction while fpc didn't
noticed it require only one instruction
both codes compiled with -O3

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440bx · 2019-04-23 14:01

Quote from: fcu on April 23, 2019, 03:49:54 pmjust looking how fpc
optimize this function compared with how gcc does
gcc seems smarter, it reduce it to one instruction while fpc didn't
noticed it require only one instruction
both codes compiled with -O3
You'll find many such cases. 

A lot of C/C++ programmers are addicted to optimizing code.  That can
be a good thing sometimes but, sometimes it isn't (no shortage of
times when one looks at a piece of C/C++ code and thinks... what the
hell is this supposed to do !?)

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marcov · 2019-04-23 14:11

This is a bit a specific case for C. Because there are so many C
compilers, and umpteen codebases all have such constructs for
endianswaps, they added a peephole.

A more logical FPC way would be to simply declare an intrinsic.

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ASerge · 2019-04-23 18:13

By the way, this is assembler code for System.SwapEndian(X)
Code: ASM  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i ;
SYSTEM_$$_SWAPENDIAN$LONGWORD$$LONGWORD mov %ecx,%eaxbswap
 %eaxretqFPC 3.0.4, x64, Windows

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fcu · 2019-04-23 21:03

Quote from: ASerge on April 23, 2019, 08:13:08 pmBy the way, this is
assembler code for System.SwapEndian(X)
Code: ASM  [Select][+][-]window.onload = function(){var x1 =
document.getElementById("main_content_section"); if (x1) { var x =
document.getElementsByClassName("geshi");for (var i = 0; i ;
SYSTEM_$$_SWAPENDIAN$LONGWORD$$LONGWORD mov %ecx,%eaxbswap  %eaxretq
what version do you have ?
i am using 3.0.4 32bits  ,and this is the asm of this function

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ASerge · 2019-04-23 22:04

Quote from: fcu on April 23, 2019, 11:03:57 pmwhat version do you have
?
i am using 3.0.4 32bits  ,and this is the asm of this function
As I wrote 64-bit. Can be 32 bit code such because the BSWAP
instruction is not supported on IA-32 processors earlier than the
Intel 486 processor family?