Successfully identified regression in *gcc* in CI configuration
tcwg_gcc_bootstrap/master-arm-bootstrap_debug. So far, this commit has
regressed CI configurations:
- tcwg_gcc_bootstrap/master-arm-bootstrap_debug
Culprit:
<cut>
commit 1d244020246cb155e4de62ca3b302b920a1f513f
Author: Roger Sayle <ro...@nextmovesoftware.com>
Date: Mon Aug 23 12:37:04 2021 +0100
Fold sign of LSHIFT_EXPR to eliminate no-op conversions.
This short patch teaches fold that it is "safe" to change the sign
of a left shift, to reduce the number of type conversions in gimple.
As an example:
unsigned int foo(unsigned int i) {
return (int)i << 8;
}
is currently optimized to:
unsigned int foo (unsigned int i)
{
int i.0_1;
int _2;
unsigned int _4;
<bb 2> [local count: 1073741824]:
i.0_1 = (int) i_3(D);
_2 = i.0_1 << 8;
_4 = (unsigned int) _2;
return _4;
}
with this patch, this now becomes:
unsigned int foo (unsigned int i)
{
unsigned int _2;
<bb 2> [local count: 1073741824]:
_2 = i_1(D) << 8;
return _2;
}
which generates exactly the same assembly language. Aside from the
reduced memory usage, the real benefit is that no-op conversions tend
to interfere with many folding optimizations. For example,
unsigned int bar(unsigned char i) {
return (i ^ (i<<16)) | (i<<8);
}
currently gets (tangled in conversions and) optimized to:
unsigned int bar (unsigned char i)
{
unsigned int _1;
unsigned int _2;
int _3;
int _4;
unsigned int _6;
unsigned int _8;
<bb 2> [local count: 1073741824]:
_1 = (unsigned int) i_5(D);
_2 = _1 * 65537;
_3 = (int) i_5(D);
_4 = _3 << 8;
_8 = (unsigned int) _4;
_6 = _2 | _8;
return _6;
}
but with this patch, bar now optimizes down to:
unsigned int bar(unsigned char i)
{
unsigned int _1;
unsigned int _4;
<bb 2> [local count: 1073741824]:
_1 = (unsigned int) i_3(D);
_4 = _1 * 65793;
return _4;
}
2021-08-23 Roger Sayle <ro...@nextmovesoftware.com>
gcc/ChangeLog
* match.pd (shift transformations): Change the sign of an
LSHIFT_EXPR if it reduces the number of explicit conversions.
gcc/testsuite/ChangeLog
* gcc.dg/fold-convlshift-1.c: New test case.
* gcc.dg/fold-convlshift-2.c: New test case.
</cut>
Results regressed to (for first_bad == 1d244020246cb155e4de62ca3b302b920a1f513f)
# reset_artifacts:
-10
# true:
0
# build_abe binutils:
1
# First few build errors in logs:
# 00:06:26 make[3]: [armv8l-unknown-linux-gnueabihf/bits/largefile-config.h]
Error 1 (ignored)
# 00:25:39 make[3]: [armv8l-unknown-linux-gnueabihf/bits/largefile-config.h]
Error 1 (ignored)
# 00:29:38
/home/tcwg-buildslave/workspace/tcwg_gnu_8/abe/snapshots/gcc.git~master/gcc/bitmap.h:357:13:
error: type mismatch in ‘lshift_expr’
# 00:29:38
/home/tcwg-buildslave/workspace/tcwg_gnu_8/abe/snapshots/gcc.git~master/gcc/bitmap.h:357:13:
internal compiler error: ‘verify_gimple’ failed
# 00:29:38 make[3]: *** [bitmap.o] Error 1
# 00:34:06 make[2]: *** [all-stage3-gcc] Error 2
# 00:34:06 make[1]: *** [stage3-bubble] Error 2
# 00:34:07 make: *** [all] Error 2
from (for last_good == b320edc0c29c838b0090c3c9be14187d132f73f2)
# reset_artifacts:
-10
# true:
0
# build_abe binutils:
1
# build_abe bootstrap_debug:
2
Artifacts of last_good build:
https://ci.linaro.org/job/tcwg_gcc_bootstrap-bisect-master-arm-bootstrap_debug/2/artifact/artifacts/build-b320edc0c29c838b0090c3c9be14187d132f73f2/
Artifacts of first_bad build:
https://ci.linaro.org/job/tcwg_gcc_bootstrap-bisect-master-arm-bootstrap_debug/2/artifact/artifacts/build-1d244020246cb155e4de62ca3b302b920a1f513f/
Build top page/logs:
https://ci.linaro.org/job/tcwg_gcc_bootstrap-bisect-master-arm-bootstrap_debug/2/
Configuration details:
Reproduce builds:
<cut>
mkdir investigate-gcc-1d244020246cb155e4de62ca3b302b920a1f513f
cd investigate-gcc-1d244020246cb155e4de62ca3b302b920a1f513f
git clone https://git.linaro.org/toolchain/jenkins-scripts
mkdir -p artifacts/manifests
curl -o artifacts/manifests/build-baseline.sh
https://ci.linaro.org/job/tcwg_gcc_bootstrap-bisect-master-arm-bootstrap_debug/2/artifact/artifacts/manifests/build-baseline.sh
--fail
curl -o artifacts/manifests/build-parameters.sh
https://ci.linaro.org/job/tcwg_gcc_bootstrap-bisect-master-arm-bootstrap_debug/2/artifact/artifacts/manifests/build-parameters.sh
--fail
curl -o artifacts/test.sh
https://ci.linaro.org/job/tcwg_gcc_bootstrap-bisect-master-arm-bootstrap_debug/2/artifact/artifacts/test.sh
--fail
chmod +x artifacts/test.sh
# Reproduce the baseline build (build all pre-requisites)
./jenkins-scripts/tcwg_gnu-build.sh @@ artifacts/manifests/build-baseline.sh
# Save baseline build state (which is then restored in artifacts/test.sh)
mkdir -p ./bisect
rsync -a --del --delete-excluded --exclude /bisect/ --exclude /artifacts/
--exclude /gcc/ ./ ./bisect/baseline/
cd gcc
# Reproduce first_bad build
git checkout --detach 1d244020246cb155e4de62ca3b302b920a1f513f
../artifacts/test.sh
# Reproduce last_good build
git checkout --detach b320edc0c29c838b0090c3c9be14187d132f73f2
../artifacts/test.sh
cd ..
</cut>
History of pending regressions and results:
https://git.linaro.org/toolchain/ci/base-artifacts.git/log/?h=linaro-local/ci/tcwg_gcc_bootstrap/master-arm-bootstrap_debug
Artifacts:
https://ci.linaro.org/job/tcwg_gcc_bootstrap-bisect-master-arm-bootstrap_debug/2/artifact/artifacts/
Build log:
https://ci.linaro.org/job/tcwg_gcc_bootstrap-bisect-master-arm-bootstrap_debug/2/consoleText
Full commit (up to 1000 lines):
<cut>
commit 1d244020246cb155e4de62ca3b302b920a1f513f
Author: Roger Sayle <ro...@nextmovesoftware.com>
Date: Mon Aug 23 12:37:04 2021 +0100
Fold sign of LSHIFT_EXPR to eliminate no-op conversions.
This short patch teaches fold that it is "safe" to change the sign
of a left shift, to reduce the number of type conversions in gimple.
As an example:
unsigned int foo(unsigned int i) {
return (int)i << 8;
}
is currently optimized to:
unsigned int foo (unsigned int i)
{
int i.0_1;
int _2;
unsigned int _4;
<bb 2> [local count: 1073741824]:
i.0_1 = (int) i_3(D);
_2 = i.0_1 << 8;
_4 = (unsigned int) _2;
return _4;
}
with this patch, this now becomes:
unsigned int foo (unsigned int i)
{
unsigned int _2;
<bb 2> [local count: 1073741824]:
_2 = i_1(D) << 8;
return _2;
}
which generates exactly the same assembly language. Aside from the
reduced memory usage, the real benefit is that no-op conversions tend
to interfere with many folding optimizations. For example,
unsigned int bar(unsigned char i) {
return (i ^ (i<<16)) | (i<<8);
}
currently gets (tangled in conversions and) optimized to:
unsigned int bar (unsigned char i)
{
unsigned int _1;
unsigned int _2;
int _3;
int _4;
unsigned int _6;
unsigned int _8;
<bb 2> [local count: 1073741824]:
_1 = (unsigned int) i_5(D);
_2 = _1 * 65537;
_3 = (int) i_5(D);
_4 = _3 << 8;
_8 = (unsigned int) _4;
_6 = _2 | _8;
return _6;
}
but with this patch, bar now optimizes down to:
unsigned int bar(unsigned char i)
{
unsigned int _1;
unsigned int _4;
<bb 2> [local count: 1073741824]:
_1 = (unsigned int) i_3(D);
_4 = _1 * 65793;
return _4;
}
2021-08-23 Roger Sayle <ro...@nextmovesoftware.com>
gcc/ChangeLog
* match.pd (shift transformations): Change the sign of an
LSHIFT_EXPR if it reduces the number of explicit conversions.
gcc/testsuite/ChangeLog
* gcc.dg/fold-convlshift-1.c: New test case.
* gcc.dg/fold-convlshift-2.c: New test case.
---
gcc/match.pd | 9 +++++++++
gcc/testsuite/gcc.dg/fold-convlshift-1.c | 20 ++++++++++++++++++++
gcc/testsuite/gcc.dg/fold-convlshift-2.c | 20 ++++++++++++++++++++
3 files changed, 49 insertions(+)
diff --git a/gcc/match.pd b/gcc/match.pd
index 0fcfd0ea62c..978a1b0172e 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -3385,6 +3385,15 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
(if (integer_zerop (@2) || integer_all_onesp (@2))
(cmp @0 @2)))))
+/* Both signed and unsigned lshift produce the same result, so use
+ the form that minimizes the number of conversions. */
+(simplify
+ (convert (lshift:s@0 (convert:s@1 @2) INTEGER_CST@3))
+ (if (tree_nop_conversion_p (type, TREE_TYPE (@0))
+ && INTEGRAL_TYPE_P (TREE_TYPE (@2))
+ && TYPE_PRECISION (TREE_TYPE (@2)) <= TYPE_PRECISION (type))
+ (lshift (convert @2) @3)))
+
/* Simplifications of conversions. */
/* Basic strip-useless-type-conversions / strip_nops. */
diff --git a/gcc/testsuite/gcc.dg/fold-convlshift-1.c
b/gcc/testsuite/gcc.dg/fold-convlshift-1.c
new file mode 100644
index 00000000000..b6f57f81e72
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/fold-convlshift-1.c
@@ -0,0 +1,20 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+unsigned int foo(unsigned int i)
+{
+ int t1 = i;
+ int t2 = t1 << 8;
+ return t2;
+}
+
+int bar(int i)
+{
+ unsigned int t1 = i;
+ unsigned int t2 = t1 << 8;
+ return t2;
+}
+
+/* { dg-final { scan-tree-dump-not "\\(int\\)" "optimized" } } */
+/* { dg-final { scan-tree-dump-not "\\(unsigned int\\)" "optimized" } } */
+
diff --git a/gcc/testsuite/gcc.dg/fold-convlshift-2.c
b/gcc/testsuite/gcc.dg/fold-convlshift-2.c
new file mode 100644
index 00000000000..f21358c4584
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/fold-convlshift-2.c
@@ -0,0 +1,20 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -fdump-tree-optimized" } */
+
+unsigned int foo(unsigned char c)
+{
+ int t1 = c;
+ int t2 = t1 << 8;
+ return t2;
+}
+
+int bar(unsigned char c)
+{
+ unsigned int t1 = c;
+ unsigned int t2 = t1 << 8;
+ return t2;
+}
+
+/* { dg-final { scan-tree-dump-times "\\(int\\)" 1 "optimized" } } */
+/* { dg-final { scan-tree-dump-times "\\(unsigned int\\)" 1 "optimized" } } */
+
</cut>
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