[PATCH] D48789: [X86] Replace (32/64 - n) shift amounts with (neg n) since the shift amount is masked in hardware

[Roman Lebedev via Phabricator via llvm-commits]

lebedev.ri added inline comments.


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Comment at: lib/Target/X86/X86ISelDAGToDAG.cpp:2698-2699
+
+  // Special case to avoid messing up a BZHI pattern.
+  // Look for (srl (shl X, (size - y)), (size - y)
+  if (Subtarget->hasBMI2() && (VT == MVT::i32 || VT == MVT::i64) &&
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Why? I would think we can simply add one more pattern to the `BZHI`?
I think this is bad because it not only not "messing up a BZHI pattern",
but anything else too, since it does not check that the user is `and %val, %mask`.


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Comment at: test/CodeGen/X86/extract-lowbits.ll:1-11
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc -mtriple=i686-unknown-linux-gnu -mattr=-bmi,-tbm,-bmi2 < %s | FileCheck %s --check-prefixes=CHECK,X86,NOBMI,X86-NOBMI
 ; RUN: llc -mtriple=i686-unknown-linux-gnu -mattr=+bmi,-tbm,-bmi2 < %s | FileCheck %s --check-prefixes=CHECK,X86,BMI1,X86-BMI1
 ; RUN: llc -mtriple=i686-unknown-linux-gnu -mattr=+bmi,+tbm,-bmi2 < %s | FileCheck %s --check-prefixes=CHECK,X86,BMI1,X86-BMI1,BMI1TBM,X86-BMI1TBM
 ; RUN: llc -mtriple=i686-unknown-linux-gnu -mattr=+bmi,+tbm,+bmi2 < %s | FileCheck %s --check-prefixes=CHECK,X86,BMI1,X86-BMI1,BMI1BMI2,X86-BMI1BMI2,BMI1TBM,X86-BMI1TBM,BMI1TBMBMI2,X86-BMI1TBMBMI2
 ; RUN: llc -mtriple=i686-unknown-linux-gnu -mattr=+bmi,-tbm,+bmi2 < %s | FileCheck %s --check-prefixes=CHECK,X86,BMI1,X86-BMI1,BMI1BMI2,X86-BMI1BMI2,BMI1NOTBMBMI2,X86-BMI1NOTBMBMI2
 ; RUN: llc -mtriple=x86_64-unknown-linux-gnu -mattr=-bmi,-tbm,-bmi2 < %s | FileCheck %s --check-prefixes=CHECK,X64,NOBMI,X64-NOBMI
----------------
I'll commit the change to this file from D48490 tomorrow.
That will expose more fold possibilities here.


https://reviews.llvm.org/D48789