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180 lines
4.0 KiB
Diff
180 lines
4.0 KiB
Diff
From f3ed8bfe8a82af1870ddc8696ed4cc1d5aa6b441 Mon Sep 17 00:00:00 2001
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From: Rich Felker <dalias@aerifal.cx>
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Date: Mon, 5 Aug 2019 18:41:47 -0400
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Subject: fix x87 stack imbalance in corner cases of i386 math asm
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commit 31c5fb80b9eae86f801be4f46025bc6532a554c5 introduced underflow
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code paths for the i386 math asm, along with checks on the fpu status
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word to skip the underflow-generation instructions if the underflow
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flag was already raised. unfortunately, at least one such path, in
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log1p, returned with 2 items on the x87 stack rather than just 1 item
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for the return value. this is a violation of the ABI's calling
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convention, and could cause subsequent floating point code to produce
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NANs due to x87 stack overflow. if floating point results are used in
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flow control, this can lead to runaway wrong code execution.
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rather than reviewing each "underflow already raised" code path for
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correctness, remove them all. they're likely slower than just
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performing the underflow code unconditionally, and significantly more
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complex.
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all of this code should be ripped out and replaced by C source files
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with inline asm. doing so would preclude this kind of error by having
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the compiler perform all x87 stack register allocation and stack
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manipulation, and would produce comparable or better code. however
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such a change is a much larger project.
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---
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src/math/i386/asin.s | 10 ++--------
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src/math/i386/atan.s | 7 ++-----
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src/math/i386/atan2.s | 5 +----
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src/math/i386/atan2f.s | 5 +----
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src/math/i386/atanf.s | 7 ++-----
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src/math/i386/exp.s | 10 ++--------
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src/math/i386/log1p.s | 7 ++-----
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src/math/i386/log1pf.s | 7 ++-----
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8 files changed, 14 insertions(+), 44 deletions(-)
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--- a/src/math/i386/asin.s
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+++ b/src/math/i386/asin.s
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@@ -7,13 +7,10 @@ asinf:
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cmp $0x01000000,%eax
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jae 1f
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# subnormal x, return x with underflow
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- fnstsw %ax
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- and $16,%ax
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- jnz 2f
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fld %st(0)
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fmul %st(1)
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fstps 4(%esp)
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-2: ret
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+ ret
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.global asinl
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.type asinl,@function
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@@ -30,11 +27,8 @@ asin:
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cmp $0x00200000,%eax
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jae 1f
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# subnormal x, return x with underflow
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- fnstsw %ax
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- and $16,%ax
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- jnz 2f
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fsts 4(%esp)
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-2: ret
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+ ret
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1: fld %st(0)
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fld1
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fsub %st(0),%st(1)
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--- a/src/math/i386/atan.s
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+++ b/src/math/i386/atan.s
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@@ -10,8 +10,5 @@ atan:
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fpatan
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ret
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# subnormal x, return x with underflow
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-1: fnstsw %ax
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- and $16,%ax
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- jnz 2f
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- fsts 4(%esp)
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-2: ret
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+1: fsts 4(%esp)
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+ ret
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--- a/src/math/i386/atan2.s
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+++ b/src/math/i386/atan2.s
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@@ -10,8 +10,5 @@ atan2:
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cmp $0x00200000,%eax
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jae 1f
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# subnormal x, return x with underflow
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- fnstsw %ax
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- and $16,%ax
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- jnz 1f
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fsts 4(%esp)
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-1: ret
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+ ret
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--- a/src/math/i386/atan2f.s
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+++ b/src/math/i386/atan2f.s
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@@ -10,10 +10,7 @@ atan2f:
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cmp $0x01000000,%eax
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jae 1f
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# subnormal x, return x with underflow
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- fnstsw %ax
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- and $16,%ax
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- jnz 1f
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fld %st(0)
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fmul %st(1)
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fstps 4(%esp)
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-1: ret
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+ ret
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--- a/src/math/i386/atanf.s
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+++ b/src/math/i386/atanf.s
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@@ -10,10 +10,7 @@ atanf:
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fpatan
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ret
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# subnormal x, return x with underflow
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-1: fnstsw %ax
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- and $16,%ax
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- jnz 2f
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- fld %st(0)
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+1: fld %st(0)
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fmul %st(1)
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fstps 4(%esp)
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-2: ret
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+ ret
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--- a/src/math/i386/exp.s
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+++ b/src/math/i386/exp.s
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@@ -7,13 +7,10 @@ expm1f:
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cmp $0x01000000,%eax
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jae 1f
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# subnormal x, return x with underflow
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- fnstsw %ax
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- and $16,%ax
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- jnz 2f
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fld %st(0)
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fmul %st(1)
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fstps 4(%esp)
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-2: ret
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+ ret
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.global expm1l
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.type expm1l,@function
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@@ -30,11 +27,8 @@ expm1:
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cmp $0x00200000,%eax
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jae 1f
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# subnormal x, return x with underflow
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- fnstsw %ax
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- and $16,%ax
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- jnz 2f
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fsts 4(%esp)
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-2: ret
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+ ret
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1: fldl2e
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fmulp
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mov $0xc2820000,%eax
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--- a/src/math/i386/log1p.s
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+++ b/src/math/i386/log1p.s
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@@ -16,9 +16,6 @@ log1p:
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fyl2x
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ret
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# subnormal x, return x with underflow
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-2: fnstsw %ax
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- and $16,%ax
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- jnz 1f
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- fsts 4(%esp)
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+2: fsts 4(%esp)
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fstp %st(1)
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-1: ret
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+ ret
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--- a/src/math/i386/log1pf.s
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+++ b/src/math/i386/log1pf.s
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@@ -16,10 +16,7 @@ log1pf:
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fyl2x
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ret
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# subnormal x, return x with underflow
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-2: fnstsw %ax
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- and $16,%ax
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- jnz 1f
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- fxch
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+2: fxch
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fmul %st(1)
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fstps 4(%esp)
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-1: ret
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+ ret
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