[fpc-devel] Quirk is "IsJumpToLabel"

[J. Gareth Moreton]

Hi everyone,

I've been developing a new optimisation for x86, and in one situation a 
JMP becomes a Jcc.  To make sure it's valid, I ensure that 
"IsJumpToLabel" returns True before the change is made.  All was well in 
x86_64-win64 and x86_64-linux, but on i386-linux, I came across a bit of 
an anomaly:

     jmp _$RTTI$_Ld3(,%eax,4)

It turns out that "IsJumpToLabel" returns true for this construct, which 
is not valid for Jcc.  _$RTTI$_Ld3 is a jump table stored as a data 
structure.  The question is though... should this be treated as a jump 
to a label?

Currently, my optimisation fails on i386-linux because of 
"IsJumpToLabel" returning True on this.  I can modify my code so it 
makes sure there's no index register, but this feels a bit hacky and 
there may be other, unrelated blocks of code that could fall foul of a 
similar situation, and I personally feel that "IsJumpToLabel" should 
return True only for pure labels.  However, such a change will affect 
other platforms and I don't yet know what effect that will have.

As a side-node, because of the principle of relocation under x86_64, 
jump table access is more complex.  The equivalent code for "jmp 
_$RTTI$_Ld3(,%eax,4)" on x86_64-win64 is:

     leaq    .Ld3(%rip),%rdx
     movslq    (%rdx,%rax,4),%rax
     addq    %rdx,%rax
     jmp    *%rax

(The jump table contains relative addresses rather than absolute 
addresses, hence the need for "addq %rdx,%rax")

And of course, the JMP instruction is not considered a jump to a label.

Kit