Peter Bigot <big...@acm.org> writes: > I'm updating the machine description for a 16-bit microcontroller, and am > trying to eliminate some hand-coded SImode and DImode insns because in > general gcc generates better code automatically. So far, I've found no case > where I seem to need to implement cbranchsi4 or cbranchdi4. > > However, gcc does generate a reference to __ucmpsi2 when compiling a switch > statement with a default clause and a 32-bit integer as the selection > expression. > > The mailing list archives suggest that cmpsi2 was at one point a library > routine, but it is not documented in the internals (at least in the 4.5.x > branch where I'm currently working). libgcc2 provides a presumably trusted > implementation of __ucmpdi2, though except for a switch statement with a > default clause and a 64-bit selection expression I've been unable to find > any way to generate test code that actually uses it > (gcc.c-torture/execute/cmpdi-1.c doesn't.) I've never been able to get the > signed version __cmpdi2 to show up. > > One past message (http://gcc.gnu.org/ml/gcc/2007-08/msg00225.html) suggests > that a similar evocation of __cmpsi2 might be a bug that arose because there > are "two different sizes of signed integers both of which are larger than > word_mode". Which is the case in the MSP430. > > GCC seems fully capable of reducing SImode and DImode comparisons to HImode > in every other situation. Is generation of this undocumented helper > function worth treating as a bug in the compiler, or is the documentation > incomplete and back-ends (or libgcc2.c) should be providing an > implementation? If the former, I'll spend some time trying to isolate it; > if the latter, I won't bother.
I believe it is a documentation bug that __cmpdi2 and __ucmpdi2 are documented but __cmpsi2 and __ucmpsi2 are not. gcc can certainly generate calls to __cmpsi2 and __ucmpsi2 in some cases for some targets. It does appear that if you have a cbranchsi4 insn in your MD file, and you don't have a conditional move instruction, then I don't think __cmpsi2 or __ucmpsi2 will ever be called. Ian
