PowerPC ABI Changes in GCC 3.4

The GCC 3.4 release series fixed several cases in which earlier releases did not follow the proper calling conventions. These fixes unfortunately did not all happen in one release, so in certain rare cases code will be incompatible when using different compiler versions within the release series.

Fixed in 3.4.0

• _Complex integer types smaller than _Complex long, and _Complex floating point types smaller than _Complex double were passed and returned incorrectly in one register instead of the real and imaginary components being in separate registers. This has been fixed for PowerPC64 GNU/Linux and AIX, with PowerPC GNU/Linux and Darwin opting to stay compatible with GCC 3.3.

• Under -mabi=altivec, PowerPC64 GNU/Linux used to pass vector arguments to non-prototyped functions only in vector registers. Without a prototype the argument is supposed to be passed in both vector registers and in general purpose registers or on the stack.

• DWARF2 debug information failed to use the correct numbers for registers other than general purpose or floating point registers. This affected both PowerPC and PowerPC64 GNU/Linux.

• Many bugs fixed related to mixed-mode code, that is, use of 64-bit instructions and data on a 32-bit ABI. Previously, 64-bit values such as long long were passed in one register instead of being split into separate 32-bit registers as required by the ABI.

• Zero size arrays were incorrectly passed by reference in GCC 3.3.1 and GCC 3.3.0, a regression from GCC 3.2. This was fixed in GCC 3.3.2.

• Small structures passed by value were passed at the wrong end of registers, or padded at the wrong end in memory, or passed on the stack when they should be passed in registers. AIX and PowerPC64 GNU/Linux support has been changed to comply with their respective ABIs, which necessarily introduces an incompatibility with GCC 3.3. "Small" here means less than eight bytes for the 64-bit ABIs, and less than four bytes for AIX-32.

Fixed in 3.4.1

• Under -mabi=no-altivec, vector parameters were not aligned, and wrong code was used to access them if the target function used vector instructions. Furthermore, -maltivec affected the way vector parameters were passed and returned. This affected all ABIs, both 32-bit and 64-bit. The fix necessarily introduces an incompatibility between GCC 3.3 and GCC 3.4, but not for the more common -mabi=altivec case.

• PowerPC GNU/Linux had a number of problems with functions accepting variable arguments, with disagreement between caller and callee over where arguments were to be found. Affected were: all _Complex types, 128-bit long double, and vectors under -mabi=altivec when all vector registers had been used.

• PowerPC64 GNU/Linux and AIX generated incorrect code for certain _Complex types in functions accepting variable arguments. _Complex integer types smaller than _Complex long, and _Complex floating point types smaller than _Complex double, were not properly repackaged from registers used to pass the values.

• A 128-bit long double partially passed in f13 (because f2 .. f12 were used for other floating point arguments) also incorrectly used f14 instead of passing the remainder of the argument on the stack. Affected PowerPC64 GNU/Linux, AIX, and Darwin.

Fixed in 3.4.2

• Arguments requiring alignment, and also passed partly in registers and partly on the stack, did not have all the stack words written. This bug affected AIX-32 and Darwin in the following cases:
  • Under -mabi=altivec, vector arguments passed in the variable argument part of a function with an ellipsis in the prototype.
  • Under -mabi=no-altivec, all vector arguments.
  To be affected by this bug, the vector argument needs to be passed in r9, r10 and two stack words, and r8 not be used for passing other arguments.

• AIX-32 didn't pass 128-bit long double correctly if it needed to be passed both in general purpose and floating point registers (variable argument functions, or not prototyped). For example, if the long double should have been passed partially in r8, r9, r10 then only r8 was set.

• A 128-bit long double passed partially in floating point registers and partially on the stack caused a compiler abort on PowerPC64 GNU/Linux and AIX-64.

• Mixed-mode parameter passing bugs fixed for PowerPC GNU/Linux. Somehow, PowerPC GNU/Linux missed the fixes applied for the other 32-bit ABIs for GCC 3.4.0.

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Last modified 2006-06-21