Date: Wed, 25 Sep 2019 18:50:57 +0000 (UTC) From: Dimitry Andric <dim@FreeBSD.org> To: src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: svn commit: r352710 - head/lib/msun/src Message-ID: <201909251850.x8PIovMr008289@repo.freebsd.org>
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Author: dim Date: Wed Sep 25 18:50:57 2019 New Revision: 352710 URL: https://svnweb.freebsd.org/changeset/base/352710 Log: Do not left-shift a negative number (inducing undefined behavior in C/C++) in exp(3), expf(3), expm1(3) and expm1f(3) during intermediate computations that compute the IEEE-754 bit pattern for |2**k| for integer |k|. The implementations of exp(3), expf(3), expm1(3) and expm1f(3) need to compute IEEE-754 bit patterns for 2**k in certain places. (k is an integer and 2**k is exactly representable in IEEE-754.) Currently they do things like 0x3FF0'0000+(k<<20), which is to say they take the bit pattern representing 1 and then add directly to the exponent field to get the desired power of two. This is fine when k is non-negative. But when k<0 (and certain classes of input trigger this), this left-shifts a negative number -- an operation with undefined behavior in C and C++. The desired semantics can be achieved by instead adding the possibly-negative k to the IEEE-754 exponent bias to get the desired exponent field, _then_ shifting that into its proper overall position. (Note that in case of s_expm1.c and s_expm1f.c, there are SET_HIGH_WORD and SET_FLOAT_WORD uses further down in each of these files that perform shift operations involving k, but by these points k's range has been restricted to 2 < k <= 56, and the shift operations under those circumstances can't do anything that would be UB.) Submitted by: Jeff Walden, https://github.com/jswalden Obtained from: https://github.com/freebsd/freebsd/pull/411 Obtained from: https://github.com/freebsd/freebsd/pull/412 MFC after: 3 days Modified: head/lib/msun/src/e_exp.c head/lib/msun/src/e_expf.c head/lib/msun/src/s_expm1.c head/lib/msun/src/s_expm1f.c Modified: head/lib/msun/src/e_exp.c ============================================================================== --- head/lib/msun/src/e_exp.c Wed Sep 25 18:48:05 2019 (r352709) +++ head/lib/msun/src/e_exp.c Wed Sep 25 18:50:57 2019 (r352710) @@ -145,9 +145,9 @@ __ieee754_exp(double x) /* default IEEE double exp */ /* x is now in primary range */ t = x*x; if(k >= -1021) - INSERT_WORDS(twopk,0x3ff00000+(k<<20), 0); + INSERT_WORDS(twopk,((u_int32_t)(0x3ff+k))<<20, 0); else - INSERT_WORDS(twopk,0x3ff00000+((k+1000)<<20), 0); + INSERT_WORDS(twopk,((u_int32_t)(0x3ff+(k+1000)))<<20, 0); c = x - t*(P1+t*(P2+t*(P3+t*(P4+t*P5)))); if(k==0) return one-((x*c)/(c-2.0)-x); else y = one-((lo-(x*c)/(2.0-c))-hi); Modified: head/lib/msun/src/e_expf.c ============================================================================== --- head/lib/msun/src/e_expf.c Wed Sep 25 18:48:05 2019 (r352709) +++ head/lib/msun/src/e_expf.c Wed Sep 25 18:50:57 2019 (r352710) @@ -83,9 +83,9 @@ __ieee754_expf(float x) /* x is now in primary range */ t = x*x; if(k >= -125) - SET_FLOAT_WORD(twopk,0x3f800000+(k<<23)); + SET_FLOAT_WORD(twopk,((u_int32_t)(0x7f+k))<<23); else - SET_FLOAT_WORD(twopk,0x3f800000+((k+100)<<23)); + SET_FLOAT_WORD(twopk,((u_int32_t)(0x7f+(k+100)))<<23); c = x - t*(P1+t*P2); if(k==0) return one-((x*c)/(c-(float)2.0)-x); else y = one-((lo-(x*c)/((float)2.0-c))-hi); Modified: head/lib/msun/src/s_expm1.c ============================================================================== --- head/lib/msun/src/s_expm1.c Wed Sep 25 18:48:05 2019 (r352709) +++ head/lib/msun/src/s_expm1.c Wed Sep 25 18:50:57 2019 (r352710) @@ -188,7 +188,7 @@ expm1(double x) e = hxs*((r1-t)/(6.0 - x*t)); if(k==0) return x - (x*e-hxs); /* c is 0 */ else { - INSERT_WORDS(twopk,0x3ff00000+(k<<20),0); /* 2^k */ + INSERT_WORDS(twopk,((u_int32_t)(0x3ff+k))<<20,0); /* 2^k */ e = (x*(e-c)-c); e -= hxs; if(k== -1) return 0.5*(x-e)-0.5; Modified: head/lib/msun/src/s_expm1f.c ============================================================================== --- head/lib/msun/src/s_expm1f.c Wed Sep 25 18:48:05 2019 (r352709) +++ head/lib/msun/src/s_expm1f.c Wed Sep 25 18:50:57 2019 (r352710) @@ -94,7 +94,7 @@ expm1f(float x) e = hxs*((r1-t)/((float)6.0 - x*t)); if(k==0) return x - (x*e-hxs); /* c is 0 */ else { - SET_FLOAT_WORD(twopk,0x3f800000+(k<<23)); /* 2^k */ + SET_FLOAT_WORD(twopk,((u_int32_t)(0x7f+k))<<23); /* 2^k */ e = (x*(e-c)-c); e -= hxs; if(k== -1) return (float)0.5*(x-e)-(float)0.5;
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