On Wed, Dec 07, 2016 at 12:40:34PM +0100, Georg-Johann Lay wrote: > ...to be more specific, attached is a C test case. > > Function mod_mul is noinline + noclone, and is called with > n=317. mod_mul itself does not use "317" anywhere. > > Yet when compiling with > > $ gcc mod.c -Os -save-temps -dp > > $ gcc --version > gcc (GCC) 7.0.0 20161202 (experimental) [trunk revision 227650] > > and reading asm, there is: > > mod_mul: > .LFB1: > .cfi_startproc > cmpl $316, %edx # 9 *cmpsi_1/1 [length = 6] > pushq %rbx # 84 *pushdi2_rex64/1 [length = 1] > .cfi_def_cfa_offset 16 > .cfi_offset 3, -16 > movl %esi, %ebx # 3 *movsi_internal/1 [length = 2] > movl %edx, %esi # 4 *movsi_internal/1 [length = 2] > jbe .L2 # 10 *jcc_1 [length = 2] > movl $1, %edx # 12 *movsi_internal/1 [length = 5] > movl $317, %edi # 14 *movsi_internal/1 [length = 5] > call mod_mul # 15 *call_value [length = 5] > > So the constant 317 made its way into the code of mod_mul (insn 14), > and insn 9 also uses this value. > > From my understanding "noclone" should avoid any such propagations? noclone prevents cloning the function, and it indeed has not been cloned. Because the function is static, GCC knows all callers, and it determines all callers have n=317, so it optimises with that knowledge. Segher
