Just a followup on this patch. We did some run-time performance testing internally on this set of change on sparc M8 machine with -mmisalign and -mno-misalign based on the latest upstream gcc
for CPU2017 C/C++ SPEED run: ***without -O, -mmisalign slowdown the run-time performance about 4% on average This is mainly due to the following workaround to misaligned support in M8: (config/sparc/sparc.c) +/* for misaligned ld/st provided by M8, the IMM field is 10-bit wide + other than the 13-bit for regular ld/st. + The best solution for this problem is to distinguish each ld/st + whether it's aligned or misaligned. However, due to the current + design of the common routine TARGET_LEGITIMATE_ADDRESS_P, only + the ADDR of a ld/st is passed to the routine, the align info + carried by the corresponding MEM is NOT passed in. without changing + the prototype of TARGET_LEGITIMATE_ADDRESS_P, we cannot use this + best solution. + as a workaround, we have to conservatively treat ALL IMM field of + a ld/st insn on a MISALIGNED target is 10-bit wide. + the side-effect of this workaround is: there will be additiona + REG<-IMM insn generated for regular ld/st when -mmisalign is ON. + However, such additional reload insns should be very easily to be + removed by a set of optimization whenever -O specified. +*/ +#define RTX_OK_FOR_OFFSET_P(X, MODE) \ + (CONST_INT_P (X) \ + && ((!TARGET_MISALIGN \ + && INTVAL (X) >=3D3D -0x1000 \ + && INTVAL (X) <=3D3D (0x1000 - GET_MODE_SIZE (MODE)))\ + || (TARGET_MISALIGN \ + && INTVAL (X) >=3D3D -0x0400 \ + && INTVAL (X) <=3D3D (0x0400 - GET_MODE_SIZE (MODE))))) due to this run-time regression introduced by this workaround is not trivial, We decided to hold on this set of change at this time. Thanks. Qing > > This set of change is to provide a way to use misaligned load/store insns to > implement the compiler-time known unaligned memory access, -mno-misalign can > be used > to disable such behavior very easily if our performance data shows that > misaligned load/store insns are slower than the current software emulation. > > Qing
