| Issue |
161342
|
| Summary |
[Clang][] `-mlink-builtin-bitcode` may change FP32 denormal mode of linked module
|
| Labels |
clang:codegen
|
| Assignees |
Maetveis
|
| Reporter |
Maetveis
|
# Description
`-mlink-builtin-bitcode` may change the FP32 denormal mode of linked module if not explicitly specified.
# Reproducer
```llvm
// a.ll
declare float @add(float, float)
define float @f() #0 {
; The constant 0x37F0000000000000 is a denormal number in FP32
; 2 x F32(0x37F0000000000000) = F32(0x380000000000000) which is still denormal
; This is the expected result. With (incorrect) flushing of denormals we will get +0.
%1 = call float @add(float 0x37F0000000000000, float 0x37F0000000000000)
ret float %1
}
attributes #0 = { "denormal-fp-math"="preserve-sign,preserve-sign" }
```
```llvm
// bc.ll
define float @add(float %a, float %b) {
%1 = fadd float %a, %b
ret float %1
}
```
Compile `a.ll` and link in the bitcode from `bc.bc`.
```console
> llvm-as bc.ll
> clang -cc1 -fdenormal-fp-math=preserve-sign,preserve-sign -mlink-builtin-bitcode bc.bc -emit-llvm -o - a.ll
```
# Observed behaviour
The attributes on `@add` have been changed to flush denormals (for F32 only):
```llvm
; (...)
define internal float @add(float %a, float %b) #1 {
; (...)
; attributes #1 = { convergent "denormal-fp-math-f32"="preserve-sign,preserve-sign" ... }
```
This changes the behaviour of the program, for example turning on optimizations produces:
```console
> clang -cc1 -O1 -fdenormal-fp-math=preserve-sign,preserve-sign -mlink-builtin-bitcode bc.bc -emit-llvm -o - a.ll
```
```llvm
; (...)
define noundef float @f() local_unnamed_addr #0 {
ret float 0.000000e+00
}
```
# Expected behaviour
`@add` should have no `denormal-fp-math(-f32)` attributes added to it, this corresponds to the default mode as specified by IEEE.
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