From 517ddcfb705fb05303e965a0ef145cd38070db7e Mon Sep 17 00:00:00 2001 From: bde Date: Wed, 13 Feb 2008 10:44:44 +0000 Subject: [PATCH] Fix exp2*(x) on signaling NaNs by returning x+x as usual. This has the side effect of confusing gcc-4.2.1's optimizer into more often doing the right thing. When it does the wrong thing here, it seems to be mainly making too many copies of x with dependency chains. This effect is tiny on amd64, but in some cases on i386 it is enormous. E.g., on i386 (A64) with -O1, the current version of exp2() should take about 50 cycles, but took 83 cycles before this change and 66 cycles after this change. exp2f() with -O1 only speeded up from 51 to 47 cycles. (exp2f() should take about 40 cycles, on an Athlon in either i386 or amd64 mode, and now takes 42 on amd64). exp2l() with -O1 slowed down from 155 cycles to 123 for some args; this is unimportant since the i386 exp2l() is a fake; the wrong thing for it seems to involve branch misprediction. --- lib/msun/ld128/s_exp2l.c | 2 +- lib/msun/ld80/s_exp2l.c | 2 +- lib/msun/src/s_exp2.c | 2 +- lib/msun/src/s_exp2f.c | 2 +- 4 files changed, 4 insertions(+), 4 deletions(-) diff --git a/lib/msun/ld128/s_exp2l.c b/lib/msun/ld128/s_exp2l.c index a94b89220737..31178e40624d 100644 --- a/lib/msun/ld128/s_exp2l.c +++ b/lib/msun/ld128/s_exp2l.c @@ -371,7 +371,7 @@ exp2l(long double x) if (u.xbits.manh != 0 || u.xbits.manl != 0 || (hx & 0x8000) == 0) - return (x); /* x is NaN or +Inf */ + return (x + x); /* x is NaN or +Inf */ else return (0.0); /* x is -Inf */ } diff --git a/lib/msun/ld80/s_exp2l.c b/lib/msun/ld80/s_exp2l.c index 8a0dbe2d3309..14dfc1d87f21 100644 --- a/lib/msun/ld80/s_exp2l.c +++ b/lib/msun/ld80/s_exp2l.c @@ -226,7 +226,7 @@ exp2l(long double x) if (ix >= BIAS + 14) { /* |x| >= 16384 or x is NaN */ if (ix == BIAS + LDBL_MAX_EXP) { if (u.xbits.man != 1ULL << 63 || (hx & 0x8000) == 0) - return (x); /* x is NaN or +Inf */ + return (x + x); /* x is +Inf or NaN */ else return (0.0); /* x is -Inf */ } diff --git a/lib/msun/src/s_exp2.c b/lib/msun/src/s_exp2.c index 63b8997cfd97..c46750f42ba0 100644 --- a/lib/msun/src/s_exp2.c +++ b/lib/msun/src/s_exp2.c @@ -351,7 +351,7 @@ exp2(double x) if(ix >= 0x7ff00000) { GET_LOW_WORD(lx,x); if(((ix & 0xfffff) | lx) != 0 || (hx & 0x80000000) == 0) - return (x); /* x is NaN or +Inf */ + return (x + x); /* x is NaN or +Inf */ else return (0.0); /* x is -Inf */ } diff --git a/lib/msun/src/s_exp2f.c b/lib/msun/src/s_exp2f.c index 1b9299f56e36..43da2f6b66f6 100644 --- a/lib/msun/src/s_exp2f.c +++ b/lib/msun/src/s_exp2f.c @@ -104,7 +104,7 @@ exp2f(float x) if(ix >= 0x43000000) { /* |x| >= 128 */ if(ix >= 0x7f800000) { if ((ix & 0x7fffff) != 0 || (hx & 0x80000000) == 0) - return (x); /* x is NaN or +Inf */ + return (x + x); /* x is NaN or +Inf */ else return (0.0); /* x is -Inf */ }