[cfe-dev] [LLVMdev] Odd PPC inline asm constraint
hfinkel at anl.gov
Fri Apr 27 18:30:24 PDT 2012
Thanks! Do you happen to know where this needs to be changed in clang
or LLVM. The code that actually interprets the constraints,
generically, is in CodeGen/SelectionDAG/TargetLowering.cpp, is clang
relying on that code, or is there some frontend code in clang itself
that is failing to initially interpret the string? If it is the code in
TargetLowering, then I don't see any support there for '*' or '#'.
On Fri, 27 Apr 2012 19:33:58 -0500
Peter Bergner <bergner at vnet.ibm.com> wrote:
> On Fri, 2012-04-27 at 14:54 -0500, Hal Finkel wrote:
> > There is a comment in the file which reads:
> > /* The weird 'i#*X' constraints on the following suppress a gcc
> > warning when __excepts is not a constant. Otherwise, they mean
> > the same as just plain 'i'. */
> > ("mtfsb0 %s0" : : "i#*X"(__builtin_ffs (__excepts)));
> > Does anyone know what that "weird" asm constraint actually means?
> The "i" and "X" constraints are documented here:
> An immediate integer operand (one with constant value) is allowed.
> This includes symbolic constants whose values will be known only
> at assembly time or later.
> Any operand whatsoever is allowed.
> The # and * constraint modifiers are documented here:
> Says that all following characters, up to the next comma, are to
> be ignored as a constraint. They are significant only for choosing
> register preferences.
> Says that the following character should be ignored when choosing
> register preferences. `*' has no effect on the meaning of the
> constraint as a constraint, and no effect on reloading.
> For more info about PowerPC specific constraints, you'll want to look
> I'll note that the "s" in the %s0 means to only print the low 5 bits
> of operand 0. I think that may only be documented in the src:
Leadership Computing Facility
Argonne National Laboratory
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