Re: [COMMITTERS] pgsql: Convert contrib/seg's bool-returning SQL functions to V1 call co

[Tom Lane]

Noah Misch  writes:
> On Fri, Apr 22, 2016 at 03:54:48PM +, Tom Lane wrote:
>> Convert contrib/seg's bool-returning SQL functions to V1 call convention.
>> 
>> It appears that we can no longer get away with using V0 call convention
>> for bool-returning functions in newer versions of MSVC.  The compiler
>> seems to generate code that doesn't clear the higher-order bits of the
>> result register, causing the bool result Datum to often read as "true"
>> when "false" was intended.  This is not very surprising, since the
>> function thinks it's returning a bool-width result but fmgr_oldstyle
>> assumes that V0 functions return "char *"; what's surprising is that
>> that hack worked for so long on so many platforms.

> Does this warrant a change to the "Section 2" comment of postgres.h,
> explaining that its precautions no longer suffice everywhere?

Hmmm ... looking at that again, and particularly at the definition
of DatumGetBool, I wonder whether the true problem is that a cast to
bool is being interpreted differently than we expect.  The intention,
as per its comment, is that we should consider only the rightmost byte
of the Datum value but consider any nonzero value of that byte as "true".
But if MSVC is now treating bool according to C99 rules, it might be
interpreting "(bool) (X)" as "(X) != 0".

It would be interesting to find out if VS2015 passes with this commit
reverted and instead defining DatumGetBool as, say,

#define DatumGetBool(X) ((bool) (GET_1_BYTE(X) != 0))

regards, tom lane


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[COMMITTERS] pgsql: Convert contrib/seg's bool-returning SQL functions to V1 call co

[Tom Lane]

Convert contrib/seg's bool-returning SQL functions to V1 call convention.

It appears that we can no longer get away with using V0 call convention
for bool-returning functions in newer versions of MSVC.  The compiler
seems to generate code that doesn't clear the higher-order bits of the
result register, causing the bool result Datum to often read as "true"
when "false" was intended.  This is not very surprising, since the
function thinks it's returning a bool-width result but fmgr_oldstyle
assumes that V0 functions return "char *"; what's surprising is that
that hack worked for so long on so many platforms.

The only functions of this description in core+contrib are in contrib/seg,
which we'd intentionally left mostly in V0 style to serve as a warning
canary if V0 call convention breaks.  We could imagine hacking things
so that they're still V0 (we'd have to redeclare the bool-returning
functions as returning some suitably wide integer type, like size_t,
at the C level).  But on the whole it seems better to convert 'em to V1.
We can still leave the pointer- and int-returning functions in V0 style,
so that the test coverage isn't gone entirely.

Back-patch to 9.5, since our intention is to support VS2015 in 9.5
and later.  There's no SQL-level change in the functions' behavior
so back-patching should be safe enough.

Discussion: <22094.1461273...@sss.pgh.pa.us>

Michael Paquier, adjusted some by me

Branch
--
REL9_5_STABLE

Details
---
http://git.postgresql.org/pg/commitdiff/b1dd2f86ce7d43f23f6aae307bb22de826849e7d

Modified Files
--
contrib/seg/seg.c | 300 +++---
1 file changed, 195 insertions(+), 105 deletions(-)


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[COMMITTERS] pgsql: Convert contrib/seg's bool-returning SQL functions to V1 call co

[Tom Lane]

Convert contrib/seg's bool-returning SQL functions to V1 call convention.

It appears that we can no longer get away with using V0 call convention
for bool-returning functions in newer versions of MSVC.  The compiler
seems to generate code that doesn't clear the higher-order bits of the
result register, causing the bool result Datum to often read as "true"
when "false" was intended.  This is not very surprising, since the
function thinks it's returning a bool-width result but fmgr_oldstyle
assumes that V0 functions return "char *"; what's surprising is that
that hack worked for so long on so many platforms.

The only functions of this description in core+contrib are in contrib/seg,
which we'd intentionally left mostly in V0 style to serve as a warning
canary if V0 call convention breaks.  We could imagine hacking things
so that they're still V0 (we'd have to redeclare the bool-returning
functions as returning some suitably wide integer type, like size_t,
at the C level).  But on the whole it seems better to convert 'em to V1.
We can still leave the pointer- and int-returning functions in V0 style,
so that the test coverage isn't gone entirely.

Back-patch to 9.5, since our intention is to support VS2015 in 9.5
and later.  There's no SQL-level change in the functions' behavior
so back-patching should be safe enough.

Discussion: <22094.1461273...@sss.pgh.pa.us>

Michael Paquier, adjusted some by me

Branch
--
master

Details
---
http://git.postgresql.org/pg/commitdiff/c8e81afc60093b199a128ccdfbb692ced8e0c9cd

Modified Files
--
contrib/seg/seg.c | 300 +++---
1 file changed, 195 insertions(+), 105 deletions(-)


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