Change check for "shift > Type::BITS" to avoid offset greater than total bits.

[amykhuang]

nexttowardf16_test is resulting in calling shift and for some reason not meeting the invariant where offset is less than bits. Change the if statement to directly check if shift - 1 meets the conditions.

[llvmbot]

@llvm/pr-subscribers-libc

Author: Amy Huang (amykhuang)

Changes

nexttowardf16_test is resulting in calling shift and for some reason not meeting the invariant where offset is less than bits. Change the if statement to directly check if shift - 1 meets the conditions.

---

Full diff: https://github.com/llvm/llvm-project/pull/133016.diff

1 Files Affected:

• (modified) libc/src/__support/FPUtil/dyadic_float.h (+1-1)

diff --git a/libc/src/__support/FPUtil/dyadic_float.h b/libc/src/__support/FPUtil/dyadic_float.h
index 16b690be08612..2378b1e594c44 100644
--- a/libc/src/__support/FPUtil/dyadic_float.h
+++ b/libc/src/__support/FPUtil/dyadic_float.h
@@ -335,7 +335,7 @@ template <size_t Bits> struct DyadicFloat {
                  .get_val();
 
     MantissaType round_mask =
-        shift > MantissaType::BITS ? 0 : MantissaType(1) << (shift - 1);
+        shift - 1 >= MantissaType::BITS ? 0 : MantissaType(1) << (shift - 1);
     MantissaType sticky_mask = round_mask - MantissaType(1);
 
     bool round_bit = !(mantissa & round_mask).is_zero();

[lntue]

It looks like some false-positive from the sanitizer? Shall we create an issue for them?

[jhuber6]

x << (y - 1) can underflow if it's zero leading to an out of bounds shift. By making the check y - 1 >= bits we guarantee that it's not zero because 0 - 1 is UINTMAX.

[llvm-ci]

LLVM Buildbot has detected a new failure on builder libc-riscv32-qemu-yocto-fullbuild-dbg running on rv32gc-qemu-system while building libc at step 4 "annotate".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/196/builds/6413

Here is the relevant piece of the build log for the reference

Step 4 (annotate) failure: 'python ../llvm-zorg/zorg/buildbot/builders/annotated/libc-linux.py ...' (failure) (timed out)
...
[       OK ] LlvmLibcSqrtTest.NormalRange (342759 ms)
Ran 2 tests.  PASS: 2  FAIL: 0
[1920/1922] Running unit test libc.test.src.math.sqrtl_test.__unit__
sh: line 1: /timer.7576: Permission denied
[==========] Running 2 tests from 1 test suite.
[ RUN      ] LlvmLibcSqrtTest.DenormalValues
[       OK ] LlvmLibcSqrtTest.DenormalValues (877336 ms)
[ RUN      ] LlvmLibcSqrtTest.NormalRange
[       OK ] LlvmLibcSqrtTest.NormalRange (411781 ms)
Ran 2 tests.  PASS: 2  FAIL: 0
command timed out: 1200 seconds without output running [b'python', b'../llvm-zorg/zorg/buildbot/builders/annotated/libc-linux.py', b'--debug'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=3767.273608
Step 8 (libc-unit-tests) failure: libc-unit-tests (failure)
...
[ RUN      ] LlvmLibcAddTestFloat.NormalRange
[       OK ] LlvmLibcAddTestFloat.NormalRange (13432 ms)
[ RUN      ] LlvmLibcAddTestLongDouble.SubnormalRange
[       OK ] LlvmLibcAddTestLongDouble.SubnormalRange (64793 ms)
[ RUN      ] LlvmLibcAddTestLongDouble.NormalRange
[       OK ] LlvmLibcAddTestLongDouble.NormalRange (81285 ms)
[ RUN      ] LlvmLibcAddTestFloat128.SubnormalRange
[       OK ] LlvmLibcAddTestFloat128.SubnormalRange (65192 ms)
[ RUN      ] LlvmLibcAddTestFloat128.NormalRange
[       OK ] LlvmLibcAddTestFloat128.NormalRange (70704 ms)
Ran 8 tests.  PASS: 8  FAIL: 0
[1917/1922] Running unit test libc.test.src.math.sqrtf128_test.__unit__
sh: line 1: /timer.7616: Permission denied
[==========] Running 3 tests from 1 test suite.
[ RUN      ] LlvmLibcSqrtTest.DenormalValues
[       OK ] LlvmLibcSqrtTest.DenormalValues (293505 ms)
[ RUN      ] LlvmLibcSqrtTest.NormalRange
[       OK ] LlvmLibcSqrtTest.NormalRange (130686 ms)
[ RUN      ] LlvmLibcSqrtTest.SpecialInputs
[       OK ] LlvmLibcSqrtTest.SpecialInputs (27 ms)
Ran 3 tests.  PASS: 3  FAIL: 0
[1918/1922] Running unit test libc.test.src.math.fsqrtl_test.__unit__
sh: line 1: /timer.8073: Permission denied
[==========] Running 2 tests from 1 test suite.
[ RUN      ] LlvmLibcSqrtTest.DenormalValues
[       OK ] LlvmLibcSqrtTest.DenormalValues (702184 ms)
[ RUN      ] LlvmLibcSqrtTest.NormalRange
[       OK ] LlvmLibcSqrtTest.NormalRange (327919 ms)
Ran 2 tests.  PASS: 2  FAIL: 0
[1919/1922] Running unit test libc.test.src.math.dsqrtl_test.__unit__
sh: line 1: /timer.8092: Permission denied
[==========] Running 2 tests from 1 test suite.
[ RUN      ] LlvmLibcSqrtTest.DenormalValues
[       OK ] LlvmLibcSqrtTest.DenormalValues (730296 ms)
[ RUN      ] LlvmLibcSqrtTest.NormalRange
[       OK ] LlvmLibcSqrtTest.NormalRange (342759 ms)
Ran 2 tests.  PASS: 2  FAIL: 0
[1920/1922] Running unit test libc.test.src.math.sqrtl_test.__unit__
sh: line 1: /timer.7576: Permission denied
[==========] Running 2 tests from 1 test suite.
[ RUN      ] LlvmLibcSqrtTest.DenormalValues
[       OK ] LlvmLibcSqrtTest.DenormalValues (877336 ms)
[ RUN      ] LlvmLibcSqrtTest.NormalRange
[       OK ] LlvmLibcSqrtTest.NormalRange (411781 ms)
Ran 2 tests.  PASS: 2  FAIL: 0

command timed out: 1200 seconds without output running [b'python', b'../llvm-zorg/zorg/buildbot/builders/annotated/libc-linux.py', b'--debug'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=3767.273608