[GHC] #8219: x86 definition of cas() is wrong
#8219: x86 definition of cas() is wrong ------------------------------------+------------------------------------- Reporter: parcs | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Runtime System | Version: 7.7 Keywords: | Operating System: Unknown/Multiple Architecture: Unknown/Multiple | Type of failure: None/Unknown Difficulty: Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | ------------------------------------+------------------------------------- On x86, `cas()` is defined in SMP.h as (paraphrasing) {{{ #!c asm ("lock\ncmpxchg %3,%1" :"=a"(o), "=m" (*p) :"0"(o), "r"(n)); }}} But *p is both read and written to, and therefore should have the '+' constraint modifier as opposed to the '=' modifier. Otherwise, the compiler is allowed to elide an earlier write to *p because it thinks that the call to `cas()` will overwrite it. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8219 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#8219: x86 definition of cas() is wrong -------------------------------------+------------------------------------ Reporter: parcs | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Runtime System | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by ezyang): Seems right. Do you have a test-case? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8219#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#8219: x86 definition of cas() is wrong -------------------------------------+------------------------------------ Reporter: parcs | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Runtime System | Version: 7.7 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by parcs): Test case: === cas.c {{{ #!c static int cas (int *p, int o, int n) { asm ("lock cmpxchg %2,%1" :"=a"(o), "=m"(*(volatile int *)p) :"0"(o), "r"(n)); return o; } int v; int main (int argc, char **argv) { /* The following assignment gets removed unless you change "=m" to "+m". */ v = 42; cas (&v, 0, 1); return 0; } return 0; }}} === Command Line {{{ $ gcc -O2 -S cas.c $ grep 42 cas.s zsh: exit 1 $ perl -i -pe 's/=m/+m/' cas.c $ gcc -O2 -S cas.c $ grep 42 cas.s movl $42, v(%rip) }}} -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8219#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
#8219: x86 definition of cas() is wrong
-------------------------------------+------------------------------------
Reporter: parcs | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Runtime System | Version: 7.7
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture: Unknown/Multiple
Type of failure: None/Unknown | Difficulty: Unknown
Test Case: | Blocked By:
Blocking: | Related Tickets:
-------------------------------------+------------------------------------
Comment (by parcs):
Sorry, my original test case is needlessly convoluted. Here's a more
straightforward one:
=== cas.c
{{{
#!c
#include
#8219: x86 definition of cas() is wrong
-------------------------------------+------------------------------------
Reporter: parcs | Owner:
Type: bug | Status: new
Priority: normal | Milestone:
Component: Runtime System | Version: 7.7
Resolution: | Keywords:
Operating System: Unknown/Multiple | Architecture: Unknown/Multiple
Type of failure: None/Unknown | Difficulty: Unknown
Test Case: | Blocked By:
Blocking: | Related Tickets:
-------------------------------------+------------------------------------
Comment (by Patrick Palka
#8219: x86 definition of cas() is wrong -------------------------------------+------------------------------------ Reporter: parcs | Owner: Type: bug | Status: closed Priority: normal | Milestone: Component: Runtime System | Version: 7.7 Resolution: fixed | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Changes (by parcs): * status: new => closed * resolution: => fixed -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8219#comment:5 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler
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