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author | Ben Gamari <bgamari.foss@gmail.com> | 2018-04-23 10:40:34 -0400 |
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committer | Ben Gamari <ben@smart-cactus.org> | 2018-04-24 09:55:01 -0400 |
commit | e7322107d3647e5d3097eeca878e036b1c98557b (patch) | |
tree | 0e0509964ab78bcedd01ab91d486ad9082e23abb /libraries | |
parent | f04ac4d4d0811f48c68fecacefb262039ee33239 (diff) | |
download | haskell-e7322107d3647e5d3097eeca878e036b1c98557b.tar.gz |
ghc-prim: Refactor and document __sync_fetch_and_nand workaround
ed6f9fb9d5a684d2159c29633159c3254cf04deb reduced the scope of this hack
to only include Clangs which actually lack __sync_fetch_and_nand.
However, this causes GHC to fail to build with -Werror on Clang due to
the lack of the -Wsync-nand warning flag. As it turns out a flag
controlling the warning is available under a different name, however.
Test Plan: Validate with Clang, GCC
Subscribers: thomie, carter
GHC Trac Issues: #9678
Differential Revision: https://phabricator.haskell.org/D4613
Diffstat (limited to 'libraries')
-rw-r--r-- | libraries/ghc-prim/cbits/atomic.c | 75 |
1 files changed, 60 insertions, 15 deletions
diff --git a/libraries/ghc-prim/cbits/atomic.c b/libraries/ghc-prim/cbits/atomic.c index b238041feb..0a471b31ad 100644 --- a/libraries/ghc-prim/cbits/atomic.c +++ b/libraries/ghc-prim/cbits/atomic.c @@ -107,7 +107,44 @@ hs_atomic_and64(StgWord x, StgWord64 val) // FetchNandByteArrayOp_Int -// Workaround for http://llvm.org/bugs/show_bug.cgi?id=8842 +// Note [__sync_fetch_and_nand usage] +// ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ +// +// The __sync_fetch_and_nand builtin is a bit of a disaster. It was introduced +// in GCC long ago with silly semantics. Specifically: +// +// *ptr = ~(tmp & value) +// +// Clang introduced the builtin with the same semantics. +// +// In GCC 4.4 the operation's semantics were rightly changed to, +// +// *ptr = ~tmp & value +// +// and the -Wsync-nand warning was added warning users of the operation about +// the change. +// +// Clang took this change as a reason to remove support for the +// builtin in 2010. Then, in 2014 Clang re-added support with the new +// semantics. However, the warning flag was given a different name +// (-Wsync-fetch-and-nand-semantics-changed) for added fun. +// +// Consequently, we are left with a bit of a mess: GHC requires GCC >4.4 +// (enforced by the FP_GCC_VERSION autoconf check), so we thankfully don't need +// to support the operation's older broken semantics. However, we need to take +// care to explicitly disable -Wsync-nand wherever possible, lest the build +// fails with -Werror. Furthermore, we need to emulate the operation when +// building with some Clang versions (shipped by some Mac OS X releases) which +// lack support for the builtin. +// +// In the words of Bob Dylan: everything is broken. +// +// See also: +// +// * https://bugs.llvm.org/show_bug.cgi?id=8842 +// * https://ghc.haskell.org/trac/ghc/ticket/9678 +// + #define CAS_NAND(x, val) \ { \ __typeof__ (*(x)) tmp = *(x); \ @@ -117,14 +154,22 @@ hs_atomic_and64(StgWord x, StgWord64 val) return tmp; \ } -// This is only provided by clang +// N.B. __has_builtin is only provided by clang #if !defined(__has_builtin) #define __has_builtin(x) 0 #endif +#if defined(__clang__) && !__has_builtin(__sync_fetch_and_nand) +#define USE_SYNC_FETCH_AND_NAND 0 +#else +#define USE_SYNC_FETCH_AND_NAND 1 +#endif + // Otherwise this fails with -Werror -#if defined(__GNUC__) && !defined(__clang__) #pragma GCC diagnostic push +#if defined(__clang__) +#pragma GCC diagnostic ignored "-Wsync-fetch-and-nand-semantics-changed" +#elif defined(__GNUC__) #pragma GCC diagnostic ignored "-Wsync-nand" #endif @@ -132,10 +177,10 @@ extern StgWord hs_atomic_nand8(StgWord x, StgWord val); StgWord hs_atomic_nand8(StgWord x, StgWord val) { -#if defined(__clang__) && __has_builtin(__sync_fetch_and_nand) - CAS_NAND((volatile StgWord8 *) x, (StgWord8) val) -#else +#if USE_SYNC_FETCH_AND_NAND return __sync_fetch_and_nand((volatile StgWord8 *) x, (StgWord8) val); +#else + CAS_NAND((volatile StgWord8 *) x, (StgWord8) val) #endif } @@ -143,10 +188,10 @@ extern StgWord hs_atomic_nand16(StgWord x, StgWord val); StgWord hs_atomic_nand16(StgWord x, StgWord val) { -#if defined(__clang__) && __has_builtin(__sync_fetch_and_nand) - CAS_NAND((volatile StgWord16 *) x, (StgWord16) val); -#else +#if USE_SYNC_FETCH_AND_NAND return __sync_fetch_and_nand((volatile StgWord16 *) x, (StgWord16) val); +#else + CAS_NAND((volatile StgWord16 *) x, (StgWord16) val); #endif } @@ -154,10 +199,10 @@ extern StgWord hs_atomic_nand32(StgWord x, StgWord val); StgWord hs_atomic_nand32(StgWord x, StgWord val) { -#if defined(__clang__) && __has_builtin(__sync_fetch_and_nand) - CAS_NAND((volatile StgWord32 *) x, (StgWord32) val); -#else +#if USE_SYNC_FETCH_AND_NAND return __sync_fetch_and_nand((volatile StgWord32 *) x, (StgWord32) val); +#else + CAS_NAND((volatile StgWord32 *) x, (StgWord32) val); #endif } @@ -166,10 +211,10 @@ extern StgWord64 hs_atomic_nand64(StgWord x, StgWord64 val); StgWord64 hs_atomic_nand64(StgWord x, StgWord64 val) { -#if defined(__clang__) && __has_builtin(__sync_fetch_and_nand) - CAS_NAND((volatile StgWord64 *) x, val); -#else +#if USE_SYNC_FETCH_AND_NAND return __sync_fetch_and_nand((volatile StgWord64 *) x, val); +#else + CAS_NAND((volatile StgWord64 *) x, val); #endif } #endif |