[LTO] Reset DiscardValueNames in optimize().

[fhahn]

libLTO parses options late, so at the moment the option is ignored. To fix that,
re-set it in optimize(), as at this point the options have been parsed. When
LTOCodeGenerator's constructor executes, the options haven't been passed
by the linker to libLTO yet.

Note that we keep the value name of %add = add.. because when the module
is imported, DiscardValueNames is still set to false (the default when building
with assertions).

I tried to improve this in libLTO, but I am not sure if there's a suitable callback when all options have been set.

[llvmbot]

@llvm/pr-subscribers-lto

Author: Florian Hahn (fhahn)

Changes

libLTO parses options late, so at the moment the option is ignored. To fix that,
re-set it in optimize(), as at this point the options have been parsed. When
LTOCodeGenerator's constructor executes, the options haven't been passed
by the linker to libLTO yet.

Note that we keep the value name of %add = add.. because when the module
is imported, DiscardValueNames is still set to false (the default when building
with assertions).

I tried to improve this in libLTO, but I am not sure if there's a suitable callback when all options have been set.

---

Full diff: https://github.com/llvm/llvm-project/pull/78705.diff

2 Files Affected:

• (modified) llvm/lib/LTO/LTOCodeGenerator.cpp (+3)
• (modified) llvm/test/tools/lto/discard-value-names.ll (+1-2)

diff --git a/llvm/lib/LTO/LTOCodeGenerator.cpp b/llvm/lib/LTO/LTOCodeGenerator.cpp
index 52d8fff14be9cec..727ab7b0db41a6d 100644
--- a/llvm/lib/LTO/LTOCodeGenerator.cpp
+++ b/llvm/lib/LTO/LTOCodeGenerator.cpp
@@ -582,6 +582,9 @@ bool LTOCodeGenerator::optimize() {
   if (!this->determineTarget())
     return false;
 
+  // libLTO parses options late, so re-set them here.
+  Context.setDiscardValueNames(LTODiscardValueNames);
+
   auto DiagFileOrErr = lto::setupLLVMOptimizationRemarks(
       Context, RemarksFilename, RemarksPasses, RemarksFormat,
       RemarksWithHotness, RemarksHotnessThreshold);
diff --git a/llvm/test/tools/lto/discard-value-names.ll b/llvm/test/tools/lto/discard-value-names.ll
index 723b0701ae22cee..04d25eaf6067c54 100644
--- a/llvm/test/tools/lto/discard-value-names.ll
+++ b/llvm/test/tools/lto/discard-value-names.ll
@@ -7,11 +7,10 @@
 
 ; The test requires asserts, as it depends on the default value for
 ; -lto-discard-value-names at the moment.
-; FIXME: -lto-discard-value-names is ignored at the moment.
 
 ; REQUIRES: asserts
 
-; DISCARD: %cmp.i = icmp
+; DISCARD: %{{[0-9]+}} = icmp
 ; DISCARD: %add = add i32
 
 ; KEEP: %cmp.i = icmp

[cachemeifyoucan]

I think the flag doesn't work because .bc is save-temp outputs that are not real outputs from libLTO but only for debugging intermediates. I am not against the quality of life improvement here.

Is this a FullLTO problem only or it affects thinLTO as well?

[cachemeifyoucan]

Should the REQUIRED to be removed?

[fhahn]

I think it is needed as otherwise the default is to discard value names. Left as is for now