A N-bit truncate is not much different than ANDing with a N-bit mask and so some simplifications done for AND can also be done for TRUNC. For example for code like this: char foo(int x, int y) { return (x & 0xffff) | y; } the mask is unneeded and the function should be equivalent to: char foo(int x, int y) { return x | y; } The simplification in this patch does exactly this, giving: foo: or.32 %r4 <- %arg1, %arg2 trunc.8 %r5 <- (32) %r4 ret.8 %r5 while previously the mask was not optimized away: foo: and.32 %r2 <- %arg1, $0xffff or.32 %r4 <- %r2, %arg2 trunc.8 %r5 <- (32) %r4 ret.8 %r5 This simplification is especially important for signed bitfields because the TRUNC+ZEXT of unsigned bitfields is simplified into an OP_AND but this is, of course, not the case for the TRUNC+SEXT of signed bitfields. Do the simplification by calling simplify_mask_or(), initialy used for OP_AND, but with the effective mask corresponding to TRUNC(x, N): $mask(N). Signed-off-by: Luc Van Oostenryck <luc.vanoostenryck@gmail.com>