Skip to content

[InstCombine] The sign bit of NaN is not preserved when folding (X <= +/-0.0) ? (0.0 - X) : X to fabs(X) #136646

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom
Closed
dtcxzyw opened this issue Apr 22, 2025 · 0 comments · Fixed by #136648
Closed

[InstCombine] The sign bit of NaN is not preserved when folding (X <= +/-0.0) ? (0.0 - X) : X to fabs(X) #136646

dtcxzyw opened this issue Apr 22, 2025 · 0 comments · Fixed by #136648
Assignees
@dtcxzyw
Labels
llvm:instcombine miscompilation

Comments

@dtcxzyw
Copy link
Member

dtcxzyw commented Apr 22, 2025

Reproducer: https://alive2.llvm.org/ce/z/2JaLU9


----------------------------------------
define half @src(half %x) {
#0:
  %gtzero = fcmp ugt half %x, 0x0000
  %negx = fsub half 0x0000, %x
  %fabs = select i1 %gtzero, half %x, half %negx
  ret half %fabs
}
=>
define half @src(half %x) nofree willreturn memory(none) {
#0:
  %fabs = fabs half %x
  ret half %fabs
}
Transformation doesn't verify!

ERROR: Value mismatch

Example:
half %x = #xfd00 (SNaN)

Source:
i1 %gtzero = #x1 (1)
half %negx = #xff00 (QNaN)
half %fabs = #xfd00 (SNaN)

Target:
half %fabs = #x7d00 (SNaN)
Source value: #xfd00 (SNaN)
Target value: #x7d00 (SNaN)

Summary:
  0 correct transformations
  1 incorrect transformations
  0 failed-to-prove transformations
  0 Alive2 errors

Related issues:
AliveToolkit/alive2#1155
#59279
@dtcxzyw dtcxzyw self-assigned this Apr 22, 2025
@dtcxzyw dtcxzyw mentioned this issue Apr 22, 2025
Merged
jyli0116 pushed a commit to jyli0116/llvm-project that referenced this issue Apr 28, 2025
@dtcxzyw @jyli0116
[InstCombine] Preserve signbit semantics of NaN with fold to fabs (ll…
2ae18f3 
…vm#136648)

As per the LangRef and IEEE 754-2008 standard, the sign bit of NaN is
preserved if there is no floating-point operation being performed.
See also
llvm@862e35e
for reference.

Alive2: https://alive2.llvm.org/ce/z/QYtEGj
Closes llvm#136646
swift-ci pushed a commit to swiftlang/llvm-project that referenced this issue Apr 29, 2025
@dtcxzyw @tstellar
[InstCombine] Preserve signbit semantics of NaN with fold to fabs (ll…
f4779c3 
…vm#136648)

As per the LangRef and IEEE 754-2008 standard, the sign bit of NaN is
preserved if there is no floating-point operation being performed.
See also
llvm@862e35e
for reference.

Alive2: https://alive2.llvm.org/ce/z/QYtEGj
Closes llvm#136646

(cherry picked from commit 3e1e406)
llvm-sync bot pushed a commit to arm/arm-toolchain that referenced this issue May 6, 2025
@dtcxzyw @github-actions
Automerge: [InstCombine] Preserve signbit semantics of NaN with fold …
daa89f1 
…to fabs (#136648)

As per the LangRef and IEEE 754-2008 standard, the sign bit of NaN is
preserved if there is no floating-point operation being performed.
See also
llvm/llvm-project@862e35e
for reference.

Alive2: https://alive2.llvm.org/ce/z/QYtEGj
Closes llvm/llvm-project#136646
IanWood1 pushed a commit to IanWood1/llvm-project that referenced this issue May 6, 2025
@dtcxzyw @IanWood1
[InstCombine] Preserve signbit semantics of NaN with fold to fabs (ll…
83cd1b1 
…vm#136648)

As per the LangRef and IEEE 754-2008 standard, the sign bit of NaN is
preserved if there is no floating-point operation being performed.
See also
llvm@862e35e
for reference.

Alive2: https://alive2.llvm.org/ce/z/QYtEGj
Closes llvm#136646
IanWood1 pushed a commit to IanWood1/llvm-project that referenced this issue May 6, 2025
@dtcxzyw @IanWood1
[InstCombine] Preserve signbit semantics of NaN with fold to fabs (ll…
9da3924 
…vm#136648)

As per the LangRef and IEEE 754-2008 standard, the sign bit of NaN is
preserved if there is no floating-point operation being performed.
See also
llvm@862e35e
for reference.

Alive2: https://alive2.llvm.org/ce/z/QYtEGj
Closes llvm#136646
IanWood1 pushed a commit to IanWood1/llvm-project that referenced this issue May 6, 2025
@dtcxzyw @IanWood1
[InstCombine] Preserve signbit semantics of NaN with fold to fabs (ll…
ed321a8 
…vm#136648)

As per the LangRef and IEEE 754-2008 standard, the sign bit of NaN is
preserved if there is no floating-point operation being performed.
See also
llvm@862e35e
for reference.

Alive2: https://alive2.llvm.org/ce/z/QYtEGj
Closes llvm#136646
Ankur-0429 pushed a commit to Ankur-0429/llvm-project that referenced this issue May 9, 2025
@dtcxzyw @Ankur-0429
[InstCombine] Preserve signbit semantics of NaN with fold to fabs (ll…
c0a4ae8 
…vm#136648)

As per the LangRef and IEEE 754-2008 standard, the sign bit of NaN is
preserved if there is no floating-point operation being performed.
See also
llvm@862e35e
for reference.

Alive2: https://alive2.llvm.org/ce/z/QYtEGj
Closes llvm#136646
llvm-sync bot pushed a commit to arm/arm-toolchain that referenced this issue May 15, 2025
@dtcxzyw @github-actions
Automerge: [InstCombine] Preserve signbit semantics of NaN with fold …
6d2e537 
…to fabs (#136648)

As per the LangRef and IEEE 754-2008 standard, the sign bit of NaN is
preserved if there is no floating-point operation being performed.
See also
llvm/llvm-project@862e35e
for reference.

Alive2: https://alive2.llvm.org/ce/z/QYtEGj
Closes llvm/llvm-project#136646

(cherry picked from commit 3e1e406)
Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
Assignees

@dtcxzyw dtcxzyw

Labels
llvm:instcombine miscompilation
Projects
None yet
Milestone
No milestone
Development

Successfully merging a pull request may close this issue.

[InstCombine] Preserve signbit semantics of NaN with fold to fabs dtcxzyw/llvm-project
1 participant
@dtcxzyw