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* [gcc(refs/users/meissner/heads/work139)] PowerPC: Do not depend on an undefined shift
@ 2023-10-16 18:23 Michael Meissner
0 siblings, 0 replies; 7+ messages in thread
From: Michael Meissner @ 2023-10-16 18:23 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:232f13ca9075f9ad33e6f62def03e3d6032462ce
commit 232f13ca9075f9ad33e6f62def03e3d6032462ce
Author: Michael Meissner <meissner@linux.ibm.com>
Date: Thu Oct 12 00:07:36 2023 -0400
PowerPC: Do not depend on an undefined shift
I was building a cross compiler to PowerPC on my x86_86 workstation with the
latest version of GCC on October 11th. I could not build the compiler on the
x86_64 system as it died in building libgcc. I looked into it, and I
discovered the compiler was recursing until it ran out of stack space. If I
build a native compiler with the same sources on a PowerPC system, it builds
fine.
I traced this down to a change made around October 10th:
commit 8f1a70a4fbcc6441c70da60d4ef6db1e5635e18a (HEAD)
Author: Jiufu Guo <guojiufu@linux.ibm.com>
Date: Tue Jan 10 20:52:33 2023 +0800
rs6000: build constant via li/lis;rldicl/rldicr
If a constant is possible left/right cleaned on a rotated value from
a negative value of "li/lis". Then, using "li/lis ; rldicl/rldicr"
to build the constant.
The code was doing a -1 << 64 which is undefined behavior because different
machines produce different results. On the x86_64 system, (-1 << 64) produces
-1 while on a PowerPC 64-bit system, (-1 << 64) produces 0. The x86_64 then
recurses until the stack runs out of space.
If I apply this patch, the compiler builds fine on both x86_64 as a PowerPC
crosss compiler and on a native PowerPC system.
2023-10-12 Michael Meissner <meissner@linux.ibm.com>
gcc/
PR target/111778
* config/rs6000/rs6000.cc (can_be_built_by_li_lis_and_rldicl): Do not
use a shift left by the number of bits in a host wide int.
Diff:
---
gcc/config/rs6000/rs6000.cc | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/gcc/config/rs6000/rs6000.cc b/gcc/config/rs6000/rs6000.cc
index cc24dd5301e3..6ffd982c7bcf 100644
--- a/gcc/config/rs6000/rs6000.cc
+++ b/gcc/config/rs6000/rs6000.cc
@@ -10371,7 +10371,17 @@ can_be_built_by_li_lis_and_rldicl (HOST_WIDE_INT c, int *shift,
to ones and then recheck it. */
int lz = clz_hwi (c);
+<<<<<<< HEAD
/* If lz == 0, the left shift is undefined. */
+=======
+ /* Different machines interpret num << shift differently if shift is at least
+ the number of bits in num's representation. It is explicitly undefined
+ behavior in the C/C++ langauges.
+
+ In particular (-1 << 64) on an x86_64 produces -1 and (-1 << 64) on a
+ 64-bit PowerPC produces 0. This difference causes a cross compiler on
+ x86_64 to recurse until it runs out of stack. */
+>>>>>>> 0016f87ad08 (PowerPC: Do not depend on an undefined shift)
if (!lz)
return false;
^ permalink raw reply [flat|nested] 7+ messages in thread
* [gcc(refs/users/meissner/heads/work139)] PowerPC: Do not depend on an undefined shift
@ 2023-10-16 18:24 Michael Meissner
0 siblings, 0 replies; 7+ messages in thread
From: Michael Meissner @ 2023-10-16 18:24 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:6b89ca6ea6649b6a549077530f050c82245aded0
commit 6b89ca6ea6649b6a549077530f050c82245aded0
Author: Michael Meissner <meissner@linux.ibm.com>
Date: Thu Oct 12 00:07:36 2023 -0400
PowerPC: Do not depend on an undefined shift
I was building a cross compiler to PowerPC on my x86_86 workstation with the
latest version of GCC on October 11th. I could not build the compiler on the
x86_64 system as it died in building libgcc. I looked into it, and I
discovered the compiler was recursing until it ran out of stack space. If I
build a native compiler with the same sources on a PowerPC system, it builds
fine.
I traced this down to a change made around October 10th:
commit 8f1a70a4fbcc6441c70da60d4ef6db1e5635e18a (HEAD)
Author: Jiufu Guo <guojiufu@linux.ibm.com>
Date: Tue Jan 10 20:52:33 2023 +0800
rs6000: build constant via li/lis;rldicl/rldicr
If a constant is possible left/right cleaned on a rotated value from
a negative value of "li/lis". Then, using "li/lis ; rldicl/rldicr"
to build the constant.
The code was doing a -1 << 64 which is undefined behavior because different
machines produce different results. On the x86_64 system, (-1 << 64) produces
-1 while on a PowerPC 64-bit system, (-1 << 64) produces 0. The x86_64 then
recurses until the stack runs out of space.
If I apply this patch, the compiler builds fine on both x86_64 as a PowerPC
crosss compiler and on a native PowerPC system.
2023-10-12 Michael Meissner <meissner@linux.ibm.com>
gcc/
PR target/111778
* config/rs6000/rs6000.cc (can_be_built_by_li_lis_and_rldicl): Do not
use a shift left by the number of bits in a host wide int.
Diff:
---
gcc/config/rs6000/rs6000.cc | 21 ---------------------
1 file changed, 21 deletions(-)
diff --git a/gcc/config/rs6000/rs6000.cc b/gcc/config/rs6000/rs6000.cc
index c23ebbfa7347..8f06b37171a3 100644
--- a/gcc/config/rs6000/rs6000.cc
+++ b/gcc/config/rs6000/rs6000.cc
@@ -10394,32 +10394,11 @@ can_be_built_by_li_lis_and_rldicl (HOST_WIDE_INT c, int *shift,
/* Leading zeros may be cleaned by rldicl with a mask. Change leading zeros
to ones and then recheck it. */
int lz = clz_hwi (c);
-<<<<<<< HEAD
-<<<<<<< HEAD
-<<<<<<< HEAD
/* If lz == 0, the left shift is undefined. */
-=======
- /* Different machines interpret num << shift differently if shift is at least
- the number of bits in num's representation. It is explicitly undefined
- behavior in the C/C++ langauges.
-
- In particular (-1 << 64) on an x86_64 produces -1 and (-1 << 64) on a
- 64-bit PowerPC produces 0. This difference causes a cross compiler on
- x86_64 to recurse until it runs out of stack. */
->>>>>>> 0016f87ad08 (PowerPC: Do not depend on an undefined shift)
if (!lz)
return false;
-=======
->>>>>>> ab63444d033 (Revert patches)
-=======
-
- /* If lz == 0, the left shift is undefined. */
- if (!lz)
- return false;
-
->>>>>>> 49fb3f85c90 (PowerPC: Do not depend on an undefined shift)
HOST_WIDE_INT unmask_c
= c | (HOST_WIDE_INT_M1U << (HOST_BITS_PER_WIDE_INT - lz));
int n;
^ permalink raw reply [flat|nested] 7+ messages in thread
* [gcc(refs/users/meissner/heads/work139)] PowerPC: Do not depend on an undefined shift
@ 2023-10-16 18:23 Michael Meissner
0 siblings, 0 replies; 7+ messages in thread
From: Michael Meissner @ 2023-10-16 18:23 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:36156709beefd312b27cabaa7602b87e1fae1c66
commit 36156709beefd312b27cabaa7602b87e1fae1c66
Author: Michael Meissner <meissner@linux.ibm.com>
Date: Thu Oct 12 00:49:35 2023 -0400
PowerPC: Do not depend on an undefined shift
I was building a cross compiler to PowerPC on my x86_86 workstation with the
latest version of GCC on October 11th. I could not build the compiler on the
x86_64 system as it died in building libgcc. I looked into it, and I
discovered the compiler was recursing until it ran out of stack space. If I
build a native compiler with the same sources on a PowerPC system, it builds
fine.
I traced this down to a change made around October 10th:
| commit 8f1a70a4fbcc6441c70da60d4ef6db1e5635e18a (HEAD)
| Author: Jiufu Guo <guojiufu@linux.ibm.com>
| Date: Tue Jan 10 20:52:33 2023 +0800
|
| rs6000: build constant via li/lis;rldicl/rldicr
|
| If a constant is possible left/right cleaned on a rotated value from
| a negative value of "li/lis". Then, using "li/lis ; rldicl/rldicr"
| to build the constant.
The code was doing a -1 << 64 which is undefined behavior because different
machines produce different results. On the x86_64 system, (-1 << 64) produces
-1 while on a PowerPC 64-bit system, (-1 << 64) produces 0. The x86_64 then
recurses until the stack runs out of space.
If I apply this patch, the compiler builds fine on both x86_64 as a PowerPC
crosss compiler and on a native PowerPC system.
2023-10-12 Michael Meissner <meissner@linux.ibm.com>
gcc/
PR target/111778
* config/rs6000/rs6000.cc (can_be_built_by_li_lis_and_rldicl): Protect
code from shifts that are undefined.
(can_be_built_by_li_lis_and_rldicr): Likewise.
(can_be_built_by_li_and_rldic): Likewise.
Diff:
---
gcc/config/rs6000/rs6000.cc | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/gcc/config/rs6000/rs6000.cc b/gcc/config/rs6000/rs6000.cc
index 2d1d379e1287..be6ae5c88bb4 100644
--- a/gcc/config/rs6000/rs6000.cc
+++ b/gcc/config/rs6000/rs6000.cc
@@ -10371,6 +10371,7 @@ can_be_built_by_li_lis_and_rldicl (HOST_WIDE_INT c, int *shift,
to ones and then recheck it. */
int lz = clz_hwi (c);
<<<<<<< HEAD
+<<<<<<< HEAD
<<<<<<< HEAD
/* If lz == 0, the left shift is undefined. */
@@ -10388,6 +10389,13 @@ can_be_built_by_li_lis_and_rldicl (HOST_WIDE_INT c, int *shift,
=======
>>>>>>> ab63444d033 (Revert patches)
+=======
+
+ /* If lz == 0, the left shift is undefined. */
+ if (!lz)
+ return false;
+
+>>>>>>> 49fb3f85c90 (PowerPC: Do not depend on an undefined shift)
HOST_WIDE_INT unmask_c
= c | (HOST_WIDE_INT_M1U << (HOST_BITS_PER_WIDE_INT - lz));
int n;
^ permalink raw reply [flat|nested] 7+ messages in thread
* [gcc(refs/users/meissner/heads/work139)] PowerPC: Do not depend on an undefined shift
@ 2023-10-12 21:54 Michael Meissner
0 siblings, 0 replies; 7+ messages in thread
From: Michael Meissner @ 2023-10-12 21:54 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:5ba61cf9168a7f408a9af24d618a55ac44b3cb96
commit 5ba61cf9168a7f408a9af24d618a55ac44b3cb96
Author: Michael Meissner <meissner@linux.ibm.com>
Date: Thu Oct 12 00:49:35 2023 -0400
PowerPC: Do not depend on an undefined shift
I was building a cross compiler to PowerPC on my x86_86 workstation with the
latest version of GCC on October 11th. I could not build the compiler on the
x86_64 system as it died in building libgcc. I looked into it, and I
discovered the compiler was recursing until it ran out of stack space. If I
build a native compiler with the same sources on a PowerPC system, it builds
fine.
I traced this down to a change made around October 10th:
| commit 8f1a70a4fbcc6441c70da60d4ef6db1e5635e18a (HEAD)
| Author: Jiufu Guo <guojiufu@linux.ibm.com>
| Date: Tue Jan 10 20:52:33 2023 +0800
|
| rs6000: build constant via li/lis;rldicl/rldicr
|
| If a constant is possible left/right cleaned on a rotated value from
| a negative value of "li/lis". Then, using "li/lis ; rldicl/rldicr"
| to build the constant.
The code was doing a -1 << 64 which is undefined behavior because different
machines produce different results. On the x86_64 system, (-1 << 64) produces
-1 while on a PowerPC 64-bit system, (-1 << 64) produces 0. The x86_64 then
recurses until the stack runs out of space.
If I apply this patch, the compiler builds fine on both x86_64 as a PowerPC
crosss compiler and on a native PowerPC system.
2023-10-12 Michael Meissner <meissner@linux.ibm.com>
gcc/
PR target/111778
* config/rs6000/rs6000.cc (can_be_built_by_li_lis_and_rldicl): Protect
code from shifts that are undefined.
(can_be_built_by_li_lis_and_rldicr): Likewise.
(can_be_built_by_li_and_rldic): Likewise.
Diff:
---
gcc/config/rs6000/rs6000.cc | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/gcc/config/rs6000/rs6000.cc b/gcc/config/rs6000/rs6000.cc
index 2d1d379e128..be6ae5c88bb 100644
--- a/gcc/config/rs6000/rs6000.cc
+++ b/gcc/config/rs6000/rs6000.cc
@@ -10371,6 +10371,7 @@ can_be_built_by_li_lis_and_rldicl (HOST_WIDE_INT c, int *shift,
to ones and then recheck it. */
int lz = clz_hwi (c);
<<<<<<< HEAD
+<<<<<<< HEAD
<<<<<<< HEAD
/* If lz == 0, the left shift is undefined. */
@@ -10388,6 +10389,13 @@ can_be_built_by_li_lis_and_rldicl (HOST_WIDE_INT c, int *shift,
=======
>>>>>>> ab63444d033 (Revert patches)
+=======
+
+ /* If lz == 0, the left shift is undefined. */
+ if (!lz)
+ return false;
+
+>>>>>>> 49fb3f85c90 (PowerPC: Do not depend on an undefined shift)
HOST_WIDE_INT unmask_c
= c | (HOST_WIDE_INT_M1U << (HOST_BITS_PER_WIDE_INT - lz));
int n;
^ permalink raw reply [flat|nested] 7+ messages in thread
* [gcc(refs/users/meissner/heads/work139)] PowerPC: Do not depend on an undefined shift
@ 2023-10-12 21:54 Michael Meissner
0 siblings, 0 replies; 7+ messages in thread
From: Michael Meissner @ 2023-10-12 21:54 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:30673dbfa939bd034e12d7998a7c5a5d15ad3f27
commit 30673dbfa939bd034e12d7998a7c5a5d15ad3f27
Author: Michael Meissner <meissner@linux.ibm.com>
Date: Thu Oct 12 00:07:36 2023 -0400
PowerPC: Do not depend on an undefined shift
I was building a cross compiler to PowerPC on my x86_86 workstation with the
latest version of GCC on October 11th. I could not build the compiler on the
x86_64 system as it died in building libgcc. I looked into it, and I
discovered the compiler was recursing until it ran out of stack space. If I
build a native compiler with the same sources on a PowerPC system, it builds
fine.
I traced this down to a change made around October 10th:
commit 8f1a70a4fbcc6441c70da60d4ef6db1e5635e18a (HEAD)
Author: Jiufu Guo <guojiufu@linux.ibm.com>
Date: Tue Jan 10 20:52:33 2023 +0800
rs6000: build constant via li/lis;rldicl/rldicr
If a constant is possible left/right cleaned on a rotated value from
a negative value of "li/lis". Then, using "li/lis ; rldicl/rldicr"
to build the constant.
The code was doing a -1 << 64 which is undefined behavior because different
machines produce different results. On the x86_64 system, (-1 << 64) produces
-1 while on a PowerPC 64-bit system, (-1 << 64) produces 0. The x86_64 then
recurses until the stack runs out of space.
If I apply this patch, the compiler builds fine on both x86_64 as a PowerPC
crosss compiler and on a native PowerPC system.
2023-10-12 Michael Meissner <meissner@linux.ibm.com>
gcc/
PR target/111778
* config/rs6000/rs6000.cc (can_be_built_by_li_lis_and_rldicl): Do not
use a shift left by the number of bits in a host wide int.
Diff:
---
gcc/config/rs6000/rs6000.cc | 10 ++++++++++
1 file changed, 10 insertions(+)
diff --git a/gcc/config/rs6000/rs6000.cc b/gcc/config/rs6000/rs6000.cc
index cc24dd5301e..6ffd982c7bc 100644
--- a/gcc/config/rs6000/rs6000.cc
+++ b/gcc/config/rs6000/rs6000.cc
@@ -10371,7 +10371,17 @@ can_be_built_by_li_lis_and_rldicl (HOST_WIDE_INT c, int *shift,
to ones and then recheck it. */
int lz = clz_hwi (c);
+<<<<<<< HEAD
/* If lz == 0, the left shift is undefined. */
+=======
+ /* Different machines interpret num << shift differently if shift is at least
+ the number of bits in num's representation. It is explicitly undefined
+ behavior in the C/C++ langauges.
+
+ In particular (-1 << 64) on an x86_64 produces -1 and (-1 << 64) on a
+ 64-bit PowerPC produces 0. This difference causes a cross compiler on
+ x86_64 to recurse until it runs out of stack. */
+>>>>>>> 0016f87ad08 (PowerPC: Do not depend on an undefined shift)
if (!lz)
return false;
^ permalink raw reply [flat|nested] 7+ messages in thread
* [gcc(refs/users/meissner/heads/work139)] PowerPC: Do not depend on an undefined shift
@ 2023-10-12 4:49 Michael Meissner
0 siblings, 0 replies; 7+ messages in thread
From: Michael Meissner @ 2023-10-12 4:49 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:49fb3f85c907777381cc5dde79dda2fc90c8dc9c
commit 49fb3f85c907777381cc5dde79dda2fc90c8dc9c
Author: Michael Meissner <meissner@linux.ibm.com>
Date: Thu Oct 12 00:49:35 2023 -0400
PowerPC: Do not depend on an undefined shift
I was building a cross compiler to PowerPC on my x86_86 workstation with the
latest version of GCC on October 11th. I could not build the compiler on the
x86_64 system as it died in building libgcc. I looked into it, and I
discovered the compiler was recursing until it ran out of stack space. If I
build a native compiler with the same sources on a PowerPC system, it builds
fine.
I traced this down to a change made around October 10th:
| commit 8f1a70a4fbcc6441c70da60d4ef6db1e5635e18a (HEAD)
| Author: Jiufu Guo <guojiufu@linux.ibm.com>
| Date: Tue Jan 10 20:52:33 2023 +0800
|
| rs6000: build constant via li/lis;rldicl/rldicr
|
| If a constant is possible left/right cleaned on a rotated value from
| a negative value of "li/lis". Then, using "li/lis ; rldicl/rldicr"
| to build the constant.
The code was doing a -1 << 64 which is undefined behavior because different
machines produce different results. On the x86_64 system, (-1 << 64) produces
-1 while on a PowerPC 64-bit system, (-1 << 64) produces 0. The x86_64 then
recurses until the stack runs out of space.
If I apply this patch, the compiler builds fine on both x86_64 as a PowerPC
crosss compiler and on a native PowerPC system.
2023-10-12 Michael Meissner <meissner@linux.ibm.com>
gcc/
PR target/111778
* config/rs6000/rs6000.cc (can_be_built_by_li_lis_and_rldicl): Protect
code from shifts that are undefined.
(can_be_built_by_li_lis_and_rldicr): Likewise.
(can_be_built_by_li_and_rldic): Likewise.
Diff:
---
gcc/config/rs6000/rs6000.cc | 29 ++++++++++++++++++++++++++---
1 file changed, 26 insertions(+), 3 deletions(-)
diff --git a/gcc/config/rs6000/rs6000.cc b/gcc/config/rs6000/rs6000.cc
index 2828f01413c..cc24dd5301e 100644
--- a/gcc/config/rs6000/rs6000.cc
+++ b/gcc/config/rs6000/rs6000.cc
@@ -10370,6 +10370,11 @@ can_be_built_by_li_lis_and_rldicl (HOST_WIDE_INT c, int *shift,
/* Leading zeros may be cleaned by rldicl with a mask. Change leading zeros
to ones and then recheck it. */
int lz = clz_hwi (c);
+
+ /* If lz == 0, the left shift is undefined. */
+ if (!lz)
+ return false;
+
HOST_WIDE_INT unmask_c
= c | (HOST_WIDE_INT_M1U << (HOST_BITS_PER_WIDE_INT - lz));
int n;
@@ -10398,6 +10403,11 @@ can_be_built_by_li_lis_and_rldicr (HOST_WIDE_INT c, int *shift,
/* Tailing zeros may be cleaned by rldicr with a mask. Change tailing zeros
to ones and then recheck it. */
int tz = ctz_hwi (c);
+
+ /* If tz == HOST_BITS_PER_WIDE_INT, the left shift is undefined. */
+ if (tz >= HOST_BITS_PER_WIDE_INT)
+ return false;
+
HOST_WIDE_INT unmask_c = c | ((HOST_WIDE_INT_1U << tz) - 1);
int n;
if (can_be_rotated_to_lowbits (~unmask_c, 15, &n)
@@ -10428,8 +10438,15 @@ can_be_built_by_li_and_rldic (HOST_WIDE_INT c, int *shift, HOST_WIDE_INT *mask)
right bits are shifted as 0's, and left 1's(and x's) are cleaned. */
int tz = ctz_hwi (c);
int lz = clz_hwi (c);
+
+ /* If lz == HOST_BITS_PER_WIDE_INT, the left shift is undefined. */
+ if (lz >= HOST_BITS_PER_WIDE_INT)
+ return false;
+
int middle_ones = clz_hwi (~(c << lz));
- if (tz + lz + middle_ones >= ones)
+ if (tz + lz + middle_ones >= ones
+ && (tz - lz) < HOST_BITS_PER_WIDE_INT
+ && tz < HOST_BITS_PER_WIDE_INT)
{
*mask = ((1LL << (HOST_BITS_PER_WIDE_INT - tz - lz)) - 1LL) << tz;
*shift = tz;
@@ -10440,7 +10457,8 @@ can_be_built_by_li_and_rldic (HOST_WIDE_INT c, int *shift, HOST_WIDE_INT *mask)
int leading_ones = clz_hwi (~c);
int tailing_ones = ctz_hwi (~c);
int middle_zeros = ctz_hwi (c >> tailing_ones);
- if (leading_ones + tailing_ones + middle_zeros >= ones)
+ if (leading_ones + tailing_ones + middle_zeros >= ones
+ && middle_zeros < HOST_BITS_PER_WIDE_INT)
{
*mask = ~(((1ULL << middle_zeros) - 1ULL) << tailing_ones);
*shift = tailing_ones + middle_zeros;
@@ -10450,10 +10468,15 @@ can_be_built_by_li_and_rldic (HOST_WIDE_INT c, int *shift, HOST_WIDE_INT *mask)
/* xx1..1xx: --> xx0..01..1xx: some 1's(following x's) are cleaned. */
/* Get the position for the first bit of successive 1.
The 24th bit would be in successive 0 or 1. */
- HOST_WIDE_INT low_mask = (1LL << 24) - 1LL;
+ HOST_WIDE_INT low_mask = (HOST_WIDE_INT_1U << 24) - HOST_WIDE_INT_1U;
int pos_first_1 = ((c & (low_mask + 1)) == 0)
? clz_hwi (c & low_mask)
: HOST_BITS_PER_WIDE_INT - ctz_hwi (~(c | low_mask));
+
+ /* Make sure the left and right shifts are defined. */
+ if (!IN_RANGE (pos_first_1, 1, HOST_BITS_PER_WIDE_INT-1))
+ return false;
+
middle_ones = clz_hwi (~c << pos_first_1);
middle_zeros = ctz_hwi (c >> (HOST_BITS_PER_WIDE_INT - pos_first_1));
if (pos_first_1 < HOST_BITS_PER_WIDE_INT
^ permalink raw reply [flat|nested] 7+ messages in thread
* [gcc(refs/users/meissner/heads/work139)] PowerPC: Do not depend on an undefined shift
@ 2023-10-12 4:07 Michael Meissner
0 siblings, 0 replies; 7+ messages in thread
From: Michael Meissner @ 2023-10-12 4:07 UTC (permalink / raw)
To: gcc-cvs
https://gcc.gnu.org/g:0016f87ad08d0218a51c091334c9372ed6dfa497
commit 0016f87ad08d0218a51c091334c9372ed6dfa497
Author: Michael Meissner <meissner@linux.ibm.com>
Date: Thu Oct 12 00:07:36 2023 -0400
PowerPC: Do not depend on an undefined shift
I was building a cross compiler to PowerPC on my x86_86 workstation with the
latest version of GCC on October 11th. I could not build the compiler on the
x86_64 system as it died in building libgcc. I looked into it, and I
discovered the compiler was recursing until it ran out of stack space. If I
build a native compiler with the same sources on a PowerPC system, it builds
fine.
I traced this down to a change made around October 10th:
commit 8f1a70a4fbcc6441c70da60d4ef6db1e5635e18a (HEAD)
Author: Jiufu Guo <guojiufu@linux.ibm.com>
Date: Tue Jan 10 20:52:33 2023 +0800
rs6000: build constant via li/lis;rldicl/rldicr
If a constant is possible left/right cleaned on a rotated value from
a negative value of "li/lis". Then, using "li/lis ; rldicl/rldicr"
to build the constant.
The code was doing a -1 << 64 which is undefined behavior because different
machines produce different results. On the x86_64 system, (-1 << 64) produces
-1 while on a PowerPC 64-bit system, (-1 << 64) produces 0. The x86_64 then
recurses until the stack runs out of space.
If I apply this patch, the compiler builds fine on both x86_64 as a PowerPC
crosss compiler and on a native PowerPC system.
2023-10-12 Michael Meissner <meissner@linux.ibm.com>
gcc/
PR target/111778
* config/rs6000/rs6000.cc (can_be_built_by_li_lis_and_rldicl): Do not
use a shift left by the number of bits in a host wide int.
Diff:
---
gcc/config/rs6000/rs6000.cc | 11 +++++++++++
1 file changed, 11 insertions(+)
diff --git a/gcc/config/rs6000/rs6000.cc b/gcc/config/rs6000/rs6000.cc
index 2828f01413c..b03913fe71b 100644
--- a/gcc/config/rs6000/rs6000.cc
+++ b/gcc/config/rs6000/rs6000.cc
@@ -10370,6 +10370,17 @@ can_be_built_by_li_lis_and_rldicl (HOST_WIDE_INT c, int *shift,
/* Leading zeros may be cleaned by rldicl with a mask. Change leading zeros
to ones and then recheck it. */
int lz = clz_hwi (c);
+
+ /* Different machines interpret num << shift differently if shift is at least
+ the number of bits in num's representation. It is explicitly undefined
+ behavior in the C/C++ langauges.
+
+ In particular (-1 << 64) on an x86_64 produces -1 and (-1 << 64) on a
+ 64-bit PowerPC produces 0. This difference causes a cross compiler on
+ x86_64 to recurse until it runs out of stack. */
+ if (!lz)
+ return false;
+
HOST_WIDE_INT unmask_c
= c | (HOST_WIDE_INT_M1U << (HOST_BITS_PER_WIDE_INT - lz));
int n;
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