hi all, I recently posted the following patch to LKML. ISTM that coccinelle is a reasonable way to find cases where the macro would be useful and beneficial, but detecting those situations is quite different than the usual cases.
What do you think ? It is perhaps simpler to think about one-file-at-a-time, that would cover 95% of uses. thanks ~jimc ---------- Forwarded message ---------- From: Jim Cromie <[email protected]> Date: Sun, Apr 8, 2012 at 4:38 PM Subject: [00/02] add BUILD_BUG_DECL assertion (for 3.4??) To: [email protected] 0001-bug.h-add-BUILD_BUG_DECL-usable-at-file-scope.patch 0002-bug.h-add-test-demo-module.patch (DONT APPLY) this patch (0001) adds new bug.h macro, BUILD_BUG_DECL(name, cond), which unlike other *BUG* assertions is usable at file scope. Its primary purpose is to enforce identical sizes of 2 separate arrays, which but for considerations of packing/padding/section, would be together in a struct. const char const *names[] = { "bart", "lisa", "homer", "marge" }; int a[] = {1,2,3,4}; int b[] = {1,2,3,5}; long d[] = {1,2}; BUILD_BUG_DECL(foo, ARRAY_SIZE(a) != ARRAY_SIZE(b)); BUILD_BUG_DECL(buz, sizeof(a) != sizeof(b)); // good BUILD_BUG_DECL(a, sizeof(a) != sizeof(d)); // ok on x32, error x64 BUILD_BUG_DECL(b, ARRAY_SIZE(a) != ARRAY_SIZE(names)); // good macro expands as: static __attribute__ ((__section__(".init.data"))) struct { int BUILD_BUG_DECL_buz[1 - 2*!!(sizeof(a) != sizeof(b))]; } BUILD_BUG_DECL_buz[0] __attribute__((unused)); I wanted to ask for this in 3.4, but see CAVEATS = its low risk (famous last words) = has many immediate bug-prevention applications For example (perhaps a bad one, I only eyeballed the tables themselves): in drivers/net/wireless/b43/tables_lpphy.c, these 2 tables are the same size. Should that be enforced ?? static const u16 lpphy_rev0_ofdm_cck_gain_table[] = {...} static const u16 lpphy_rev1_ofdm_cck_gain_table[] = {...} Whether or not this example is appropriate, I think its tautological that there are pairs of arrays in the code that must match on length for proper operation; this would enforce them, with trivial patches. = other *BUG* assertions use do{}while, so they cant work at file scope. BUILD_BUG_DECL is declarative, so it does work at file scope. It declares an unused 0 length array in __initdata, so shouldnt create storage. I sent a similar patch previously as part of dynamic-debug patches, Jason Baron liked it, Rusty Russell thought it was redundant and not neccessary in that context. I dont think I adequately explained (or even mentioned) the file-scope aspect. Im not cc'g them, theyre both *busy* atm. CAVEATS Macro includes __attribute__((unused)), but it seems ineffective. I also tried deref, but compiler (cc (GCC) 4.6.3 20120306 (Red Hat 4.6.3-2)) warned about it, so I yanked it. Macro does seem to create storage, so I put it in __initdata. However, referencing it from a non--init function doesnt give a compiler warning. Further, calling that function after boot gives a paging-request error, and traceback. See patch 0002. IIUC, this is expected, since __initdata has been dropped after boot is complete. What I dont get is why the compiler allowed the references; I've seen __initdata/section warnings for similar problems in the past. thanks Jim Cromie _______________________________________________ Cocci mailing list [email protected] http://lists.diku.dk/mailman/listinfo/cocci (Web access from inside DIKUs LAN only)
