https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104914
--- Comment #16 from YunQiang Su <syq at gcc dot gnu.org> --- (In reply to Roger Sayle from comment #15) > Is MIPS64 actually a TRULY_NOOP_TRUNCATION_TARGET? If SImode is implicitly > assumed to be (sign?) extended, then an arbitrary DImode value/register > can't be used as an SImode value without appropriately setting/clearing the > upper bits. > i.e. thus this integer truncation isn't a no-op. > in gcc/config/mips/mips.cc, there are lines: static bool mips_truly_noop_truncation (poly_uint64 outprec, poly_uint64 inprec) { return !TARGET_64BIT || inprec <= 32 || outprec > 32; } So for mips_truly_noop_truncation(64, 32), it is true, aka we can convert 32bit value to 64bit value without any insn. This setting is based on that most (if not all) word (32bit) operation insns are all sign-extend. For example, when we run these instructions on a MIPS64 CPU li $a1, 0x7fffffff add $a3, $a1, $a1 The result of $a3 will be: 0xffffffff fffffffe And for theses instructions: li $a1, 0x7fffffff dadd $a3, $a1, $a1 # note, add -> dadd Then the content of $a3 will be: 0x00000000 fffffffe And MIPS has the single instruction for: branch less than zero, for both MIPS32, MIPS64. Let me explain example 1: if the code is running on a 32bit CPU, the result of $a3 will be 0xfffffffe, which is -2. if the code is running on a 64bit CPU, since the result of $a3 will be sign-extend to 0xffffffff fffffffe, it is still -2. That's how MIPS make 32bit binaries run smoothly on a 64bit CPU without any mode switch. > I suspect that the underlying problem is that the backend is relying on > implicit invariants, not explicitly represented in the RTL, and then > surprised when valid RTL transformations don't preserve those > invariants/assumptions. > > I wonder why the zero_extract followed by sign_extend example mentioned in > https://gcc.gnu.org/pipermail/gcc-patches/2023-August/626137.html isn't > already being considered as a try_combine candidate, allowing the backend to > simply recognize or split it. I'll investigate. Thanks.