Build failure with v4.9-rc1 and GCC trunk -- compiler weirdness

On Wednesday, October 19, 2016 4:27:51 PM CEST Ard Biesheuvel wrote:

quoted

Why not turn it into a runtime warning in this driver?

diff --git a/drivers/clk/mvebu/armada-37xx-periph.c b/drivers/clk/mvebu/armada-37xx-periph.c
index cecb0fdfaef6..711d1d9842cc 100644
--- a/drivers/clk/mvebu/armada-37xx-periph.c
+++ b/drivers/clk/mvebu/armada-37xx-periph.c

@@ -349,8 +349,10 @@ static int armada_3700_add_composite_clk(const struct clk_periph_data *data,
                        rate->reg = reg + (u64)rate->reg;
                        for (clkt = rate->table; clkt->div; clkt++)
                                table_size++;
-                       rate->width = order_base_2(table_size);
-                       rate->lock = lock;
+                       if (!WARN_ON(table_size == 0)) {
+                               rate->width = order_base_2(table_size);
+                               rate->lock = lock;
+                       }
                }
        }

I guess Will is not looking for a way to fix the driver, but for a way
to eliminate this issue entirely going forward.

In general, I think the issue where constant folding results in
ilog2() or other similar functions being called with invalid build
time constant parameter values is simply something we have to deal
with.

In this case, it is in fact order_base_2() that deviates from its
documented behavior (as Will points out), and fixing /that/ should
make this particular issue go away afaict.

Ah, right. I also noticed that order_base_2() is defined as
log2(1 << (log2(n-1)+1)), which seems a bit redundant.
Maybe we can simplify it to something like

#define order_base_2(n) ((n) <= 1) ? 0 : log2((n) - 1) + 1)

	Arnd