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

Siddhesh Poyarekar siddhesh@sourceware.org
Tue Oct 21 18:02:39 GMT 2025 

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>
---
 math/auto-libm-test-in      |  3 ++
 math/auto-libm-test-out-pow | 65 +++++++++++++++++++++++++++++++++++++
 sysdeps/x86_64/fpu/e_powl.S | 56 +++++++++++++++++---------------
 3 files changed, 97 insertions(+), 27 deletions(-)

diff --git a/math/auto-libm-test-in b/math/auto-libm-test-in
index 1397d317fb..89a6760639 100644
--- a/math/auto-libm-test-in
+++ b/math/auto-libm-test-in
@@ -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
diff --git a/math/auto-libm-test-out-pow b/math/auto-libm-test-out-pow
index 09ec53e49e..cbca46cd0c 100644
--- a/math/auto-libm-test-out-pow
+++ b/math/auto-libm-test-out-pow
@@ -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
diff --git a/sysdeps/x86_64/fpu/e_powl.S b/sysdeps/x86_64/fpu/e_powl.S
index 620ef765a7..39f77480e8 100644
--- a/sysdeps/x86_64/fpu/e_powl.S
+++ b/sysdeps/x86_64/fpu/e_powl.S
@@ -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
-
-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
+	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
+
+	/* We come here with the stack as RES : <something>, so pop off
+	   <something>.  */
+6:	fstp	%st(1)
 	LDBL_CHECK_FORCE_UFLOW_NONNAN
 	ret
 
-- 
2.51.0

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