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@llvm/pr-subscribers-backend-amdgpu
@llvm/pr-subscribers-backend-aarch64

@llvm/pr-subscribers-llvm-globalisel

Author: Pragyansh Chaturvedi (r41k0u)

diff --git a/llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp b/llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp
index 974fc40de6222..df1b325fa5baf 100644
--- a/llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/GISelValueTracking.cpp
@@ -697,6 +697,14 @@ void GISelValueTracking::computeKnownBitsImpl(Register R, KnownBits &Known,
     }
     break;
   }
+  case TargetOpcode::G_ABS: {
+    Register SrcReg = MI.getOperand(1).getReg();
+    computeKnownBitsImpl(SrcReg, Known, DemandedElts, Depth + 1);
+    Known = Known.abs();
+    Known.Zero.setHighBits(computeNumSignBits(SrcReg, DemandedElts, Depth + 1) -
+                           1);
+    break;
+  }
   }
 
   LLVM_DEBUG(dumpResult(MI, Known, Depth));

I ran the tests and found no new regressions. I am not sure how to add tests for this (as I saw in other PRs attached to the issue). I came across update_givaluetracking_test_checks.py as the test files in other PRs seem generated. If they are required, please point me towards a test writing guide and I'll add them to this PR as well.

I ran the tests and found no new regressions. I am not sure how to add tests for this (as I saw in other PRs attached to the issue). I came across update_givaluetracking_test_checks.py as the test files in other PRs seem generated.

The output is generated, the input is not. That's a new style to directly check the output. The older style is to write the simplest code that will optimize or not based on the analysis

If they are required, please point me towards a test writing guide and I'll add them to this PR as well.

https://llvm.org/docs/TestingGuide.html#generating-assertions-in-regression-tests

You're probably best off copy-pasting one of the tests from the recent GIValueTracking commits as a starting point

Hi @davemgreen, I've added a couple of tests for this. Please lmk if more of them are needed.
I also goofed up by not creating a new branch for this PR, so once this is approved I'll rebase this against main.

I came across another issue while working on this, I will add a fix for that (and I must add a sign extension test case as well).

I have added more tests and a fix for the cache assertion. If you think I should move it to GISelValueTrackingPrinterPass::run, something like this:

assert(VTA.ComputeKnownBitsCache.empty() && "Cache should have been cleared");
KnownBits Known = VTA.getKnownBits(Reg);
unsigned SignedBits = VTA.computeNumSignBits(Reg);
ComputeKnownBitsCache.clear();

instead of what I have done right now, please let me know and I'll change it.

✅ With the latest revision this PR passed the C/C++ code formatter.

The cache assertion in getKnownBits looks like a temporary thing until the function is fleshed out (although you'd know more about this). And since it is being added to computeNumSignBits as well, I wonder if that function can benefit from the cache somehow as well.

I put up #157352 to see what people think of removing it.