Hi all, I'm seeing an arm64 build failure with -rc1 and GCC trunk, although I believe that the new compiler behaviour at the heart of the problem has the potential to affect other architectures and other pieces of kernel code relying on dead-code elimination to remove deliberately undefined functions.
The failure looks like: | drivers/built-in.o: In function `armada_3700_add_composite_clk': | | linux/drivers/clk/mvebu/armada-37xx-periph.c:351: | undefined reference to `____ilog2_NaN' | | linux/drivers/clk/mvebu/armada-37xx-periph.c:351:(.text+0xc72e0): | relocation truncated to fit: R_AARCH64_CALL26 against undefined symbol | `____ilog2_NaN' | | make: *** [vmlinux] Error 1 and if we look at the source for armada_3700_add_composite_clk, we see that this is caused by: int table_size = 0; rate->reg = reg + (u64)rate->reg; for (clkt = rate->table; clkt->div; clkt++) table_size++; rate->width = order_base_2(table_size); order_base_2 calls ilog2, which has the ____ilog2_NaN call: #define ilog2(n) \ ( \ __builtin_constant_p(n) ? ( \ (n) < 1 ? ____ilog2_NaN() : \ This is because we're in a curious case where GCC has emitted a special-cased version of armada_3700_add_composite_clk, with table_size effectively constant-folded as 0. Whilst we shouldn't see this in a non-buggy kernel (hence the deliberate call to the undefined function ____ilog2_NaN), it means that the final link fails because we have a ____ilog2_NaN in the code, with a runtime check on table_size. In other words, __builtin_constant_p appears to be weaker than we've been assuming. Talking to the compiler guys here, this is due to the "jump-threading" optimisation pass, so the patch below disables that. A simpler example is: int foo(); int bar(); int count(int *argc) { int table_size = 0; for (; *argc; argc++) table_size++; if (__builtin_constant_p(table_size)) return table_size == 0 ? foo() : bar(); return bar(); } which compiles to: count: ldr w0, [x0] cbz w0, .L4 b bar .p2align 3 .L4: b foo and, with the "optimisation" disabled: count: b bar Thoughts? It feels awfully fragile disabling passes like this, but with GCC transforming the code like this, I can't immediately think of a way to preserve the intended behaviour of the code. Will --->8 diff --git a/Makefile b/Makefile index 512e47a53e9a..750873d6d11e 100644 --- a/Makefile +++ b/Makefile @@ -641,6 +641,11 @@ endif # Tell gcc to never replace conditional load with a non-conditional one KBUILD_CFLAGS += $(call cc-option,--param=allow-store-data-races=0) +# Stop gcc from converting switches into a form that defeats dead code +# elimination and can subsequently lead to calls to intentionally +# undefined functions appearing in the final link. +KBUILD_CFLAGS += $(call cc-option,--param=max-fsm-thread-path-insns=1) + include scripts/Makefile.gcc-plugins ifdef CONFIG_READABLE_ASM