Hello, I came across a problem with the x86 backend of gcc (3.3 - 4.1). When I compile code which access a lookup table of size 65536, gcc generates code which uses 16bit opcodes.
The intel architecture optimization guide warns that 16bit opcodes should be avoided whenever possible due to Length-Changing Prefixes (LCP). The recommendation is: ""If imm16 is needed, load equivalent imm32 into a register and use the word value in the register instead. "" I would expect gcc to use a 32bits register and mask it, if needed. Even when I use 32bits registers and mask them ( & 0xFFFF), gcc detects correctly that my variable is effectively only 16bits and it generates 16bits code. I modified the generated assembly to use 32bit registers and it ran much faster. -- Cheers, Nadav
