91 lines
3.1 KiB
Diff
91 lines
3.1 KiB
Diff
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From a7bfe0a2832e66f3d732e4210b5d53c81cc7a85f Mon Sep 17 00:00:00 2001
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From: Linus Torvalds <torvalds@linux-foundation.org>
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Date: Thu, 27 Sep 2018 19:16:56 -0400
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Subject: [PATCH] give up on gcc ilog2() constant optimizations
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commit 474c90156c8dcc2fa815e6716cc9394d7930cb9c upstream.
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gcc-7 has an "optimization" pass that completely screws up, and
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generates the code expansion for the (impossible) case of calling
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ilog2() with a zero constant, even when the code gcc compiles does not
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actually have a zero constant.
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And we try to generate a compile-time error for anybody doing ilog2() on
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a constant where that doesn't make sense (be it zero or negative). So
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now gcc7 will fail the build due to our sanity checking, because it
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created that constant-zero case that didn't actually exist in the source
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code.
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There's a whole long discussion on the kernel mailing about how to work
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around this gcc bug. The gcc people themselevs have discussed their
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"feature" in
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=72785
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but it's all water under the bridge, because while it looked at one
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point like it would be solved by the time gcc7 was released, that was
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not to be.
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So now we have to deal with this compiler braindamage.
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And the only simple approach seems to be to just delete the code that
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tries to warn about bad uses of ilog2().
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So now "ilog2()" will just return 0 not just for the value 1, but for
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any non-positive value too.
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It's not like I can recall anybody having ever actually tried to use
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this function on any invalid value, but maybe the sanity check just
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meant that such code never made it out in public.
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[js] no tools/include/linux/log2.h copy of that yet
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Reported-by: Laura Abbott <labbott@redhat.com>
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Cc: John Stultz <john.stultz@linaro.org>,
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Cc: Thomas Gleixner <tglx@linutronix.de>
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Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
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Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
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Signed-off-by: Jiri Slaby <jslaby@suse.cz>
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---
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include/linux/log2.h | 13 ++-----------
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1 file changed, 2 insertions(+), 11 deletions(-)
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diff --git a/include/linux/log2.h b/include/linux/log2.h
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index fd7ff3d91..f38fae23b 100644
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--- a/include/linux/log2.h
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+++ b/include/linux/log2.h
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@@ -15,12 +15,6 @@
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#include <linux/types.h>
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#include <linux/bitops.h>
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-/*
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- * deal with unrepresentable constant logarithms
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- */
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-extern __attribute__((const, noreturn))
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-int ____ilog2_NaN(void);
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-
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/*
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* non-constant log of base 2 calculators
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* - the arch may override these in asm/bitops.h if they can be implemented
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@@ -85,7 +79,7 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
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#define ilog2(n) \
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( \
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__builtin_constant_p(n) ? ( \
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- (n) < 1 ? ____ilog2_NaN() : \
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+ (n) < 2 ? 0 : \
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(n) & (1ULL << 63) ? 63 : \
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(n) & (1ULL << 62) ? 62 : \
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(n) & (1ULL << 61) ? 61 : \
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@@ -148,10 +142,7 @@ unsigned long __rounddown_pow_of_two(unsigned long n)
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(n) & (1ULL << 4) ? 4 : \
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(n) & (1ULL << 3) ? 3 : \
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(n) & (1ULL << 2) ? 2 : \
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- (n) & (1ULL << 1) ? 1 : \
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- (n) & (1ULL << 0) ? 0 : \
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- ____ilog2_NaN() \
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- ) : \
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+ 1 ) : \
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(sizeof(n) <= 4) ? \
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__ilog2_u32(n) : \
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__ilog2_u64(n) \
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