[ValueTracking] Fix bit width handling in computeKnownBits() for GEPs (#125532)

For GEPs, we have three bit widths involved: The pointer bit width, the
index bit width, and the bit width of the GEP operands.

The correct behavior here is:
* We need to sextOrTrunc the GEP operand to the index width *before*
multiplying by the scale.
* If the index width and pointer width differ, GEP only ever modifies
the low bits. Adds should not overflow into the high bits.

I'm testing this via unit tests because it's a bit tricky to test in IR
with InstCombine canonicalization getting in the way.

1 files changed, 6 insertions, 6 deletions

diff --git a/llvm/unittests/Analysis/ValueTrackingTest.cpp b/llvm/unittests/Analysis/ValueTrackingTest.cpp
index 39865fa..50e5e0e 100644
--- a/llvm/unittests/Analysis/ValueTrackingTest.cpp
+++ b/llvm/unittests/Analysis/ValueTrackingTest.cpp
@@ -2680,7 +2680,7 @@ TEST_F(ComputeKnownBitsTest, ComputeKnownBitsAbsoluteSymbol) {
 }
 
 TEST_F(ComputeKnownBitsTest, ComputeKnownBitsGEPExtendBeforeMul) {
-  // FIXME: The index should be extended before multiplying with the scale.
+  // The index should be extended before multiplying with the scale.
   parseAssembly(R"(
     target datalayout = "p:16:16:16"
 
@@ -2692,12 +2692,12 @@ TEST_F(ComputeKnownBitsTest, ComputeKnownBitsGEPExtendBeforeMul) {
     }
     )");
   KnownBits Known = computeKnownBits(A, M->getDataLayout());
-  EXPECT_EQ(~64 & 0x7fff, Known.Zero);
-  EXPECT_EQ(64, Known.One);
+  EXPECT_EQ(~320 & 0x7fff, Known.Zero);
+  EXPECT_EQ(320, Known.One);
 }
 
 TEST_F(ComputeKnownBitsTest, ComputeKnownBitsGEPOnlyIndexBits) {
-  // FIXME: GEP should only affect the index width.
+  // GEP should only affect the index width.
   parseAssembly(R"(
     target datalayout = "p:16:16:16:8"
 
@@ -2710,8 +2710,8 @@ TEST_F(ComputeKnownBitsTest, ComputeKnownBitsGEPOnlyIndexBits) {
     }
     )");
   KnownBits Known = computeKnownBits(A, M->getDataLayout());
-  EXPECT_EQ(0x7eff, Known.Zero);
-  EXPECT_EQ(0x100, Known.One);
+  EXPECT_EQ(0x7fff, Known.Zero);
+  EXPECT_EQ(0, Known.One);
 }
 
 TEST_F(ValueTrackingTest, HaveNoCommonBitsSet) {