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Simplify powl computation for small integral y [BZ #33411]

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The powl implementation for x86_64 ends up multiplying X once more than
necessary and then throwing away that result.  This results in an
overflow flag being set in cases where there is no overflow.

Simplify the relevant portion by special casing the -3 to 3 range and
simply multiplying repetitively.

Resolves: BZ #33411
Signed-off-by: Siddhesh Poyarekar <siddhesh@sourceware.org>
Reviewed by: Paul Zimmermann <Paul.Zimmermann@inria.fr>
This commit is contained in:
Siddhesh Poyarekar
2025-10-10 20:21:13 -04:00
parent 87e850fb26
commit 1b657c53c2
3 changed files with 96 additions and 26 deletions
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3
math/auto-libm-test-in
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@@ -8372,6 +8372,9 @@ pow 0x1.059c76p+0 0x1.ff80bep+11
pow 0x1.7ac7cp+5 23
pow -0x1.7ac7cp+5 23
# BZ33411. xfail for binary32 due to BZ#33563.
pow 0x1p+8192 1.0 xfail:binary32
pown 0 0
pown 0 -0
pown -0 0

65
math/auto-libm-test-out-pow
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@@ -44221,3 +44221,68 @@ pow -0x1.7ac7cp+5 23
= pow tonearest ibm128 -0x2.f58f8p+4 0x1.7p+4 : -0xf.fffff29cf02eeec4a7cde7b5a4p+124 : inexact-ok
= pow towardzero ibm128 -0x2.f58f8p+4 0x1.7p+4 : -0xf.fffff29cf02eeec4a7cde7b5ap+124 : inexact-ok
= pow upward ibm128 -0x2.f58f8p+4 0x1.7p+4 : -0xf.fffff29cf02eeec4a7cde7b5ap+124 : inexact-ok
pow 0x1p+8192 1.0 xfail:binary32
= pow downward binary32 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow tonearest binary32 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow towardzero binary32 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow upward binary32 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow downward binary64 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow tonearest binary64 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow towardzero binary64 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow upward binary64 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow downward intel96 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow tonearest intel96 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow towardzero intel96 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow upward intel96 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow downward m68k96 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow tonearest m68k96 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow towardzero m68k96 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow upward m68k96 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow downward binary128 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow tonearest binary128 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow towardzero binary128 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow upward binary128 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow downward ibm128 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow tonearest ibm128 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow towardzero ibm128 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow upward ibm128 0xf.fffffp+124 0x1p+0 : 0xf.fffffp+124 : xfail:binary32 inexact-ok
= pow downward binary64 0xf.ffffffffffff8p+1020 0x1p+0 : 0xf.ffffffffffff8p+1020 : xfail:binary32 inexact-ok
= pow tonearest binary64 0xf.ffffffffffff8p+1020 0x1p+0 : 0xf.ffffffffffff8p+1020 : xfail:binary32 inexact-ok
= pow towardzero binary64 0xf.ffffffffffff8p+1020 0x1p+0 : 0xf.ffffffffffff8p+1020 : xfail:binary32 inexact-ok
= pow upward binary64 0xf.ffffffffffff8p+1020 0x1p+0 : 0xf.ffffffffffff8p+1020 : xfail:binary32 inexact-ok
= pow downward intel96 0xf.ffffffffffff8p+1020 0x1p+0 : 0xf.ffffffffffff8p+1020 : xfail:binary32 inexact-ok
= pow tonearest intel96 0xf.ffffffffffff8p+1020 0x1p+0 : 0xf.ffffffffffff8p+1020 : xfail:binary32 inexact-ok
= pow towardzero intel96 0xf.ffffffffffff8p+1020 0x1p+0 : 0xf.ffffffffffff8p+1020 : xfail:binary32 inexact-ok
= pow upward intel96 0xf.ffffffffffff8p+1020 0x1p+0 : 0xf.ffffffffffff8p+1020 : xfail:binary32 inexact-ok
= pow downward m68k96 0xf.ffffffffffff8p+1020 0x1p+0 : 0xf.ffffffffffff8p+1020 : xfail:binary32 inexact-ok
= pow tonearest m68k96 0xf.ffffffffffff8p+1020 0x1p+0 : 0xf.ffffffffffff8p+1020 : xfail:binary32 inexact-ok
= pow towardzero m68k96 0xf.ffffffffffff8p+1020 0x1p+0 : 0xf.ffffffffffff8p+1020 : xfail:binary32 inexact-ok
= pow upward m68k96 0xf.ffffffffffff8p+1020 0x1p+0 : 0xf.ffffffffffff8p+1020 : xfail:binary32 inexact-ok
= pow downward binary128 0xf.ffffffffffff8p+1020 0x1p+0 : 0xf.ffffffffffff8p+1020 : xfail:binary32 inexact-ok
= pow tonearest binary128 0xf.ffffffffffff8p+1020 0x1p+0 : 0xf.ffffffffffff8p+1020 : xfail:binary32 inexact-ok
= pow towardzero binary128 0xf.ffffffffffff8p+1020 0x1p+0 : 0xf.ffffffffffff8p+1020 : xfail:binary32 inexact-ok
= pow upward binary128 0xf.ffffffffffff8p+1020 0x1p+0 : 0xf.ffffffffffff8p+1020 : xfail:binary32 inexact-ok
= pow downward ibm128 0xf.ffffffffffff8p+1020 0x1p+0 : 0xf.ffffffffffff8p+1020 : xfail:binary32 inexact-ok
= pow tonearest ibm128 0xf.ffffffffffff8p+1020 0x1p+0 : 0xf.ffffffffffff8p+1020 : xfail:binary32 inexact-ok
= pow towardzero ibm128 0xf.ffffffffffff8p+1020 0x1p+0 : 0xf.ffffffffffff8p+1020 : xfail:binary32 inexact-ok
= pow upward ibm128 0xf.ffffffffffff8p+1020 0x1p+0 : 0xf.ffffffffffff8p+1020 : xfail:binary32 inexact-ok
= pow downward intel96 0x1p+8192 0x1p+0 : 0x1p+8192 : xfail:binary32 inexact-ok
= pow tonearest intel96 0x1p+8192 0x1p+0 : 0x1p+8192 : xfail:binary32 inexact-ok
= pow towardzero intel96 0x1p+8192 0x1p+0 : 0x1p+8192 : xfail:binary32 inexact-ok
= pow upward intel96 0x1p+8192 0x1p+0 : 0x1p+8192 : xfail:binary32 inexact-ok
= pow downward m68k96 0x1p+8192 0x1p+0 : 0x1p+8192 : xfail:binary32 inexact-ok
= pow tonearest m68k96 0x1p+8192 0x1p+0 : 0x1p+8192 : xfail:binary32 inexact-ok
= pow towardzero m68k96 0x1p+8192 0x1p+0 : 0x1p+8192 : xfail:binary32 inexact-ok
= pow upward m68k96 0x1p+8192 0x1p+0 : 0x1p+8192 : xfail:binary32 inexact-ok
= pow downward binary128 0x1p+8192 0x1p+0 : 0x1p+8192 : xfail:binary32 inexact-ok
= pow tonearest binary128 0x1p+8192 0x1p+0 : 0x1p+8192 : xfail:binary32 inexact-ok
= pow towardzero binary128 0x1p+8192 0x1p+0 : 0x1p+8192 : xfail:binary32 inexact-ok
= pow upward binary128 0x1p+8192 0x1p+0 : 0x1p+8192 : xfail:binary32 inexact-ok
= pow downward binary128 0xf.ffffffffffffbffffffffffffcp+1020 0x1p+0 : 0xf.ffffffffffffbffffffffffffcp+1020 : xfail:binary32 inexact-ok
= pow tonearest binary128 0xf.ffffffffffffbffffffffffffcp+1020 0x1p+0 : 0xf.ffffffffffffbffffffffffffcp+1020 : xfail:binary32 inexact-ok
= pow towardzero binary128 0xf.ffffffffffffbffffffffffffcp+1020 0x1p+0 : 0xf.ffffffffffffbffffffffffffcp+1020 : xfail:binary32 inexact-ok
= pow upward binary128 0xf.ffffffffffffbffffffffffffcp+1020 0x1p+0 : 0xf.ffffffffffffbffffffffffffcp+1020 : xfail:binary32 inexact-ok
= pow downward ibm128 0xf.ffffffffffffbffffffffffffcp+1020 0x1p+0 : 0xf.ffffffffffffbffffffffffffcp+1020 : xfail:binary32 inexact-ok
= pow tonearest ibm128 0xf.ffffffffffffbffffffffffffcp+1020 0x1p+0 : 0xf.ffffffffffffbffffffffffffcp+1020 : xfail:binary32 inexact-ok
= pow towardzero ibm128 0xf.ffffffffffffbffffffffffffcp+1020 0x1p+0 : 0xf.ffffffffffffbffffffffffffcp+1020 : xfail:binary32 inexact-ok
= pow upward ibm128 0xf.ffffffffffffbffffffffffffcp+1020 0x1p+0 : 0xf.ffffffffffffbffffffffffffcp+1020 : xfail:binary32 inexact-ok

54
sysdeps/x86_64/fpu/e_powl.S
View File

@@ -144,39 +144,41 @@ ENTRY(__ieee754_powl)
fcomip %st(1), %st // 4 : y : x
fstp %st(0) // y : x
jnc 3f
mov -8(%rsp),%eax
mov -4(%rsp),%edx
orl $0, %edx
/* Here onwards, it's just integral y in range [-3, 3]. */
movq -8(%rsp),%rax
orq $0, %rax
fstp %st(0) // x
jns 4f // y >= 0, jump
fdivrl MO(one) // 1/x (now referred to as x)
negl %eax
adcl $0, %edx
negl %edx
negq %rax
4: fldl MO(one) // 1 : x
fxch
/* If y is even, take the absolute value of x. Otherwise,
ensure all intermediate values that might overflow have the
sign of x. */
/* y range is further reduced to [0, 3]. Simply walk through the
options. First up, 0 and 1. */
test %eax, %eax
jz 6f
fxch // x : 1
subl $1, %eax
jz 6f
/* Finally, y == 2 and 3. For y == 3 we do |x| * x * |x| because x * x
and |x| * |x| decay faster towards infinity compared to x * |x|. */
fld %st // x : x : 1
fabs // |x| : x : 1
fxch // x : |x| : 1
fld %st(1) // |x| : x : |x| : 1
testb $1, %al
jnz 6f
fabs
jz 7f
fmulp %st(2) // x : |x| * |x| : 1
fstp %st(0) // |x| * |x| : 1
jmp 6f
7: fmulp // |x| * x : |x| : 1
fmulp // |x| * x * |x| : 1
6: shrdl $1, %edx, %eax
jnc 5f
fxch
fabs
fmul %st(1) // x : ST*x
fxch
5: fld %st // x : x : ST*x
fabs // |x| : x : ST*x
fmulp // |x|*x : ST*x
shrl $1, %edx
movl %eax, %ecx
orl %edx, %ecx
jnz 6b
fstp %st(0) // ST*x
/* We come here with the stack as RES : <something>, so pop off
<something>. */
6: fstp %st(1)
LDBL_CHECK_FORCE_UFLOW_NONNAN
ret