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[InstCombine] Shift amount reassociation in shifty sign bit test (PR43595)

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Summary:
This problem consists of several parts:
* Basic sign bit extraction - `trunc? (?shr %x, (bitwidth(x)-1))`.
  This is trivial, and easy to do, we have a fold for it.
* Shift amount reassociation - if we have two identical shifts,
  and we can simplify-add their shift amounts together,
  then we likely can just perform them as a single shift.
  But this is finicky, has one-use restrictions,
  and shift opcodes must be identical.

But there is a super-pattern where both of these work together.
to produce sign bit test from two shifts + comparison.
We do indeed already handle this in most cases.
But since we get that fold transitively, it has one-use restrictions.
And what's worse, in this case the right-shifts aren't required to be
identical, and we can't handle that transitively:

If the total shift amount is bitwidth-1, only a sign bit will remain
in the output value. But if we look at this from the perspective of
two shifts, we can't fold - we can't possibly know what bit pattern
we'd produce via two shifts, it will be *some* kind of a mask
produced from original sign bit, but we just can't tell it's shape:
https://rise4fun.com/Alive/cM0 https://rise4fun.com/Alive/9IN

But it will *only* contain sign bit and zeros.
So from the perspective of sign bit test, we're good:
https://rise4fun.com/Alive/FRz https://rise4fun.com/Alive/qBU
Superb!

So the simplest solution is to extend `reassociateShiftAmtsOfTwoSameDirectionShifts()` to also have a
sudo-analysis mode that will ignore extra-uses, and will only check
whether a) those are two right shifts and b) they end up with bitwidth(x)-1
shift amount and return either the original value that we sign-checking,
or null.

This does not have any functionality change for
the existing `reassociateShiftAmtsOfTwoSameDirectionShifts()`.

All that being said, as disscussed in the review, this yet again
increases usage of instsimplify in instcombine as utility.
Some day that may need to be reevaluated.

https://bugs.llvm.org/show_bug.cgi?id=43595

Reviewers: spatel, efriedma, vsk

Reviewed By: spatel

Subscribers: xbolva00, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D68930

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@375371 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Roman Lebedev
2019-10-20 19:38:50 +00:00
parent 7bd6e166d9
commit 32d24a3249
4 changed files with 66 additions and 32 deletions
Download Patch File Download Diff File Expand all files Collapse all files
test/Transforms/InstCombine/sign-bit-test-via-right-shifting-all-other-bits.ll
+6 -6
View File
@@ -45,7 +45,7 @@ define i1 @highest_bit_test_via_lshr_with_truncation(i64 %data, i32 %nbits) {
; CHECK-NEXT: call void @use32(i32 [[HIGH_BITS_EXTRACTED_NARROW]])
; CHECK-NEXT: call void @use32(i32 [[SKIP_ALL_BITS_TILL_SIGNBIT]])
; CHECK-NEXT: call void @use32(i32 [[SIGNBIT]])
; CHECK-NEXT: [[ISNEG:%.*]] = icmp ne i32 [[SIGNBIT]], 0
; CHECK-NEXT: [[ISNEG:%.*]] = icmp slt i64 [[DATA]], 0
; CHECK-NEXT: ret i1 [[ISNEG]]
;
%num_low_bits_to_skip = sub i32 64, %nbits
@@ -107,7 +107,7 @@ define i1 @highest_bit_test_via_ashr_with_truncation(i64 %data, i32 %nbits) {
; CHECK-NEXT: call void @use32(i32 [[HIGH_BITS_EXTRACTED_NARROW]])
; CHECK-NEXT: call void @use32(i32 [[SKIP_ALL_BITS_TILL_SIGNBIT]])
; CHECK-NEXT: call void @use32(i32 [[SIGNBIT]])
; CHECK-NEXT: [[ISNEG:%.*]] = icmp ne i32 [[SIGNBIT]], 0
; CHECK-NEXT: [[ISNEG:%.*]] = icmp slt i64 [[DATA]], 0
; CHECK-NEXT: ret i1 [[ISNEG]]
;
%num_low_bits_to_skip = sub i32 64, %nbits
@@ -138,7 +138,7 @@ define i1 @highest_bit_test_via_lshr_ashr(i32 %data, i32 %nbits) {
; CHECK-NEXT: call void @use32(i32 [[HIGH_BITS_EXTRACTED]])
; CHECK-NEXT: call void @use32(i32 [[SKIP_ALL_BITS_TILL_SIGNBIT]])
; CHECK-NEXT: call void @use32(i32 [[SIGNBIT]])
; CHECK-NEXT: [[ISNEG:%.*]] = icmp ne i32 [[SIGNBIT]], 0
; CHECK-NEXT: [[ISNEG:%.*]] = icmp slt i32 [[DATA]], 0
; CHECK-NEXT: ret i1 [[ISNEG]]
;
%num_low_bits_to_skip = sub i32 32, %nbits
@@ -169,7 +169,7 @@ define i1 @highest_bit_test_via_lshr_ashe_with_truncation(i64 %data, i32 %nbits)
; CHECK-NEXT: call void @use32(i32 [[HIGH_BITS_EXTRACTED_NARROW]])
; CHECK-NEXT: call void @use32(i32 [[SKIP_ALL_BITS_TILL_SIGNBIT]])
; CHECK-NEXT: call void @use32(i32 [[SIGNBIT]])
; CHECK-NEXT: [[ISNEG:%.*]] = icmp ne i32 [[SIGNBIT]], 0
; CHECK-NEXT: [[ISNEG:%.*]] = icmp slt i64 [[DATA]], 0
; CHECK-NEXT: ret i1 [[ISNEG]]
;
%num_low_bits_to_skip = sub i32 64, %nbits
@@ -200,7 +200,7 @@ define i1 @highest_bit_test_via_ashr_lshr(i32 %data, i32 %nbits) {
; CHECK-NEXT: call void @use32(i32 [[HIGH_BITS_EXTRACTED]])
; CHECK-NEXT: call void @use32(i32 [[SKIP_ALL_BITS_TILL_SIGNBIT]])
; CHECK-NEXT: call void @use32(i32 [[SIGNBIT]])
; CHECK-NEXT: [[ISNEG:%.*]] = icmp ne i32 [[SIGNBIT]], 0
; CHECK-NEXT: [[ISNEG:%.*]] = icmp slt i32 [[DATA]], 0
; CHECK-NEXT: ret i1 [[ISNEG]]
;
%num_low_bits_to_skip = sub i32 32, %nbits
@@ -231,7 +231,7 @@ define i1 @highest_bit_test_via_ashr_lshr_with_truncation(i64 %data, i32 %nbits)
; CHECK-NEXT: call void @use32(i32 [[HIGH_BITS_EXTRACTED_NARROW]])
; CHECK-NEXT: call void @use32(i32 [[SKIP_ALL_BITS_TILL_SIGNBIT]])
; CHECK-NEXT: call void @use32(i32 [[SIGNBIT]])
; CHECK-NEXT: [[ISNEG:%.*]] = icmp ne i32 [[SIGNBIT]], 0
; CHECK-NEXT: [[ISNEG:%.*]] = icmp slt i64 [[DATA]], 0
; CHECK-NEXT: ret i1 [[ISNEG]]
;
%num_low_bits_to_skip = sub i32 64, %nbits