CMOVNE strangeness..

[Ficko]

I thought share a new experience - too me at least  :lol - about this little bustard.

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xor esi, esi
xor edx, edx
cmp esi, edx
cmovne edx, dword ptr [esi]


Will generate an exception despite that the mov should not take place. :dazzled:

[clive]

It's probably doing the test late in the pipeline. What CPUs have you tested this on?

It's mentioned here on page 16 as a hazard
http://www.cs.cmu.edu/~fp/courses/15213-s07/misc/asm64-handout.pdf

[qWord]

AMD64 Architecture Programmer's Manual Volume 3, General Purpose and System Instructions:

For the memory-based forms of CMOVcc, memory-related exceptions may be reported even if the condition is false

[dedndave]

how to turn a bug into a feature:
describe it in the documentation   :bg

[Ficko]

What CPUs have you tested this on?

I3-2100

For the memory-based forms of CMOVcc, memory-related exceptions may be reported even if the condition is false

That's what I guessed but the word "may" suggest it is a random behavior most more likelly a design bug than a "feature" like Dave have hinted it. :wink

[qWord]

Of course, both, Intel and AMD, doesn't though about this problem before introducing these pipline-friendly instructions - they should also work with invalid pointers  ::)

[jj2007]

Of course, both, Intel and AMD, doesn't though about this problem before introducing these pipline-friendly instructions - they should also work with invalid pointers  ::)

Make them work with invalid pointers :bg

include \masm32\MasmBasic\MasmBasic.inc   ; download
  Init tc con   ; initialise with try/catch handling, output to console
  ErrLines      ; use error lines
  Try
   mov esi, 400010h
   .Repeat
      mov edx, -1163005939   ; set a magic value
      mov ecx, esi   ; make sure they
      sub esi, 4   ; are not equal
      SetErrLine "cmovne failed"
      cmp esi, ecx   ; never equal, so this
      cmove edx, dword ptr [esi]   ; will never be triggered
      PrintLine "edx=", Hex$(edx), " at ", Hex$(esi)
      SetErrLine "before until"
   .Until LastEx(code)   ; repeat until we get an exception
      SetErrLine "after until"
  Catch
     Print CrLf$, "Last exception info = '", LastEx(info), "', triggered at ", Hex$(LastEx(addr))
  Exit
  TryCatchEnd
end start

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Last exception info = 'cmovne failed', triggered at 00401059