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Commit6b1a213

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New method for preventing compile-time calculation of degree constants.
Commit65abaab tried to prevent the scaling constants used inthe degree-based trig functions from being precomputed at compile time,because some compilers do that with functions that don't yield resultsidentical-to-the-last-bit to what you get at runtime. A report fromPeter Eisentraut suggests that some recent compilers are smart enoughto see through that trick, though. Instead, let's put the inputs tothese calculations into non-const global variables, which should be amore reliable way of convincing the compiler that it can't assume thatthey are compile-time constants. (If we really get desperate, we couldmark these variables "volatile", but I do not believe we should have to.)
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‎src/backend/utils/adt/float.c

Lines changed: 27 additions & 22 deletions
Original file line numberDiff line numberDiff line change
@@ -77,15 +77,24 @@ static float8 atan_1_0 = 0;
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staticfloat8tan_45=0;
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staticfloat8cot_45=0;
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/*
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* These are intentionally not static; don't "fix" them. They will never
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* be referenced by other files, much less changed; but we don't want the
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* compiler to know that, else it might try to precompute expressions
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* involving them. See comments for init_degree_constants().
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*/
86+
float8degree_c_thirty=30.0;
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float8degree_c_forty_five=45.0;
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float8degree_c_sixty=60.0;
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float8degree_c_one_half=0.5;
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float8degree_c_one=1.0;
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/* Local function prototypes */
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staticintfloat4_cmp_internal(float4a,float4b);
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staticintfloat8_cmp_internal(float8a,float8b);
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staticdoublesind_q1(doublex);
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staticdoublecosd_q1(doublex);
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/* This is INTENTIONALLY NOT STATIC. Don't "fix" it. */
87-
voidinit_degree_constants(float8thirty,float8forty_five,float8sixty,
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float8one_half,float8one);
97+
staticvoidinit_degree_constants(void);
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#ifndefHAVE_CBRT
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/*
@@ -1814,35 +1823,31 @@ dtan(PG_FUNCTION_ARGS)
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* compilers out there that will precompute expressions such as sin(constant)
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* using a sin() function different from what will be used at runtime. If we
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* want exact results, we must ensure that none of the scaling constants used
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* in the degree-based trig functions are computed that way.
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*
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* The whole approach fails if init_degree_constants() gets inlined into the
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* call sites, since then constant-folding can happen anyway. Currently it
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* seems sufficient to declare it non-static to prevent that. We have no
1822-
* expectation that other files will call this, but don't tell gcc that.
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* in the degree-based trig functions are computed that way. To do so, we
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* compute them from the variables degree_c_thirty etc, which are also really
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* constants, but the compiler cannot assume that.
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*
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* Other hazards we are trying to forestall with this kluge include the
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* possibility that compilers will rearrange the expressions, or compute
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* some intermediate results in registers wider than a standard double.
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*/
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void
1829-
init_degree_constants(float8thirty,float8forty_five,float8sixty,
1830-
float8one_half,float8one)
1831-
{
1832-
sin_30=sin(thirty*RADIANS_PER_DEGREE);
1833-
one_minus_cos_60=1.0-cos(sixty*RADIANS_PER_DEGREE);
1834-
asin_0_5=asin(one_half);
1835-
acos_0_5=acos(one_half);
1836-
atan_1_0=atan(one);
1837-
tan_45=sind_q1(forty_five) /cosd_q1(forty_five);
1838-
cot_45=cosd_q1(forty_five) /sind_q1(forty_five);
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staticvoid
1835+
init_degree_constants(void)
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{
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sin_30=sin(degree_c_thirty*RADIANS_PER_DEGREE);
1838+
one_minus_cos_60=1.0-cos(degree_c_sixty*RADIANS_PER_DEGREE);
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asin_0_5=asin(degree_c_one_half);
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acos_0_5=acos(degree_c_one_half);
1841+
atan_1_0=atan(degree_c_one);
1842+
tan_45=sind_q1(degree_c_forty_five) /cosd_q1(degree_c_forty_five);
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cot_45=cosd_q1(degree_c_forty_five) /sind_q1(degree_c_forty_five);
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degree_consts_set= true;
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}
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#defineINIT_DEGREE_CONSTANTS() \
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do { \
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if (!degree_consts_set) \
1845-
init_degree_constants(30.0, 45.0, 60.0, 0.5, 1.0); \
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init_degree_constants(); \
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} while(0)
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