[Converted from Gnats 410] Below is a reply from an e-mail I sent to gcc@. This was apparently discovered ~two years ago. I'm not sure any current src/sim simulators suffer this problem. I'm mainly adding it for reference. GDB, is also very much exposed to the same problem. Release: unknown How-To-Repeat: To: Andrew Cagney CC: gcc@ Subject: Re: -fstrict-aliasing and naughty code? From: Geoff Keating Hello, > > I'm trying to understand how to write ``bad'' (host dependant) code > that doesn't get screwed by strict aliasing. > > For instance, the code snipit: > > > unsigned i; > > unsigned64 tmp_reg, tmp_reg1; > > for (i = 0; i < 4; i++) > > *( (i < 2 ? (unsigned32 *) &tmp_reg > > : (unsigned32 *) &tmp_reg1) > > + (1 - i % 2) ) = ...; > > cpu->registers[...] = tmp_reg; > > > > I'm told, is bad because: > > > apparently, when -fstrict-aliasing is in effect, gcc is > > allowed to assume that the expression inside the for loop > > has no effect on the value of tmp_reg and tmp_reg1, since > > the assignment is to an object of dissimilar type. > > Provided I make (wild?) assumptions about the host and compiler, can I > instead write the above to use something like: > > union { > unsigned64 u64; > unsigned32 u32[2]; > } tmp_reg, tmp_reg1; > > for (i = 0; i < 4; i++) > if (i < 2) > tmp_reg.u32[1 - i % 2] = ... > else > tmp_reg1.u32[1 - i %2] = ...; > cpu->registers[...] = tmp_reg.u64; Yes, this is documented to work: The practice of reading from a different union member than the one most recently written to (called "type-punning") is common. Even with `-fstrict-aliasing', type-punning is allowed, provided the memory is accessed through the union type. However, it will be no more efficient than the more portable unsigned32 tmp_reg[2], tmp_reg1[2]; for (i = 0; i < 4; i++) if (i < 2) tmp_reg[1 - i % 2] = ... else tmp_reg1[1 - i %2] = ...; cpu->registers[...] = (unsigned64)tmp_reg[0] << 32 | tmp_reg[1]; in fact it will usually be less efficient because GCC will allocate registers better for the second example.
i'm guessing you mean the code in sim/mips/interp.c:sim_fetch_register() ... *(unsigned64*)memory = H2T_8 ((unsigned32) (cpu->fgr[rn - FGR_BASE])); ...