[llvm] KnownBits: refine KnownBits::srem for high-bits (PR #109121)
Ramkumar Ramachandra via llvm-commits
llvm-commits at lists.llvm.org
Wed Sep 18 12:26:04 PDT 2024
https://github.com/artagnon updated https://github.com/llvm/llvm-project/pull/109121
>From 9014d984fbd1d9974bfbffb981eb4873b04920df Mon Sep 17 00:00:00 2001
From: Ramkumar Ramachandra <ramkumar.ramachandra at codasip.com>
Date: Tue, 17 Sep 2024 16:38:32 +0100
Subject: [PATCH 1/2] ValueTracking/test: cover known-high-bits of rem
There is an underlying bug in KnownBits, and we should theoretically be
able to determine the high-bits of an srem as shown in the test, just
like urem. In preparation to fix this bug, add pre-commit tests testing
high-bits of srem and urem.
---
...wnbits-rem-lowbits.ll => knownbits-rem.ll} | 26 +++++++++++++++++++
1 file changed, 26 insertions(+)
rename llvm/test/Analysis/ValueTracking/{knownbits-rem-lowbits.ll => knownbits-rem.ll} (85%)
diff --git a/llvm/test/Analysis/ValueTracking/knownbits-rem-lowbits.ll b/llvm/test/Analysis/ValueTracking/knownbits-rem.ll
similarity index 85%
rename from llvm/test/Analysis/ValueTracking/knownbits-rem-lowbits.ll
rename to llvm/test/Analysis/ValueTracking/knownbits-rem.ll
index 0521c7130055fe..f6ca6dfa729b7c 100644
--- a/llvm/test/Analysis/ValueTracking/knownbits-rem-lowbits.ll
+++ b/llvm/test/Analysis/ValueTracking/knownbits-rem.ll
@@ -12,6 +12,17 @@ define i8 @urem_low_bits_know(i8 %xx, i8 %yy) {
ret i8 %r
}
+define i8 @urem_high_bits_know(i8 %xx, i8 %yy) {
+; CHECK-LABEL: @urem_high_bits_know(
+; CHECK-NEXT: ret i8 0
+;
+ %x = and i8 %xx, 2
+ %y = and i8 %yy, -4
+ %rem = urem i8 %x, %y
+ %r = and i8 %rem, 8
+ ret i8 %r
+}
+
define i8 @urem_low_bits_know2(i8 %xx, i8 %yy) {
; CHECK-LABEL: @urem_low_bits_know2(
; CHECK-NEXT: ret i8 2
@@ -80,6 +91,21 @@ define i8 @srem_low_bits_know(i8 %xx, i8 %yy) {
ret i8 %r
}
+define i8 @srem_high_bits_know(i8 %xx, i8 %yy) {
+; CHECK-LABEL: @srem_high_bits_know(
+; CHECK-NEXT: [[X:%.*]] = or i8 [[XX:%.*]], -2
+; CHECK-NEXT: [[Y:%.*]] = and i8 [[YY:%.*]], -4
+; CHECK-NEXT: [[REM:%.*]] = srem i8 [[X]], [[Y]]
+; CHECK-NEXT: [[R:%.*]] = and i8 [[REM]], 8
+; CHECK-NEXT: ret i8 [[R]]
+;
+ %x = or i8 %xx, -2
+ %y = and i8 %yy, -4
+ %rem = srem i8 %x, %y
+ %r = and i8 %rem, 8
+ ret i8 %r
+}
+
define i8 @srem_low_bits_know2(i8 %xx, i8 %yy) {
; CHECK-LABEL: @srem_low_bits_know2(
; CHECK-NEXT: ret i8 1
>From 649247c86fd639e16620baf8545ddc9f076cfdd1 Mon Sep 17 00:00:00 2001
From: Ramkumar Ramachandra <ramkumar.ramachandra at codasip.com>
Date: Tue, 17 Sep 2024 16:02:54 +0100
Subject: [PATCH 2/2] KnownBits: refine KnownBits::srem for high-bits
KnownBits::srem does not correctly set the leader zero-bits, omitting
the fact that LHS may be known-negative or known-non-negative. Fix this.
Alive2 proof: https://alive2.llvm.org/ce/z/WcDkSX
---
llvm/lib/Support/KnownBits.cpp | 10 +++++++---
.../Analysis/ValueTracking/knownbits-rem.ll | 6 +-----
llvm/test/CodeGen/ARM/select-imm.ll | 17 +++++++----------
3 files changed, 15 insertions(+), 18 deletions(-)
diff --git a/llvm/lib/Support/KnownBits.cpp b/llvm/lib/Support/KnownBits.cpp
index 8e31e0ced2d731..5ec78dbb113319 100644
--- a/llvm/lib/Support/KnownBits.cpp
+++ b/llvm/lib/Support/KnownBits.cpp
@@ -1075,9 +1075,13 @@ KnownBits KnownBits::srem(const KnownBits &LHS, const KnownBits &RHS) {
// The sign bit is the LHS's sign bit, except when the result of the
// remainder is zero. The magnitude of the result should be less than or
- // equal to the magnitude of the LHS. Therefore any leading zeros that exist
- // in the left hand side must also exist in the result.
- Known.Zero.setHighBits(LHS.countMinLeadingZeros());
+ // equal to the magnitude of either operand.
+ if (LHS.isNegative() && Known.isNonZero())
+ Known.One.setHighBits(
+ std::max(LHS.countMinLeadingOnes(), RHS.countMinLeadingOnes()));
+ else if (LHS.isNonNegative())
+ Known.Zero.setHighBits(
+ std::max(LHS.countMinLeadingZeros(), RHS.countMinLeadingZeros()));
return Known;
}
diff --git a/llvm/test/Analysis/ValueTracking/knownbits-rem.ll b/llvm/test/Analysis/ValueTracking/knownbits-rem.ll
index f6ca6dfa729b7c..4f60c41c581323 100644
--- a/llvm/test/Analysis/ValueTracking/knownbits-rem.ll
+++ b/llvm/test/Analysis/ValueTracking/knownbits-rem.ll
@@ -93,11 +93,7 @@ define i8 @srem_low_bits_know(i8 %xx, i8 %yy) {
define i8 @srem_high_bits_know(i8 %xx, i8 %yy) {
; CHECK-LABEL: @srem_high_bits_know(
-; CHECK-NEXT: [[X:%.*]] = or i8 [[XX:%.*]], -2
-; CHECK-NEXT: [[Y:%.*]] = and i8 [[YY:%.*]], -4
-; CHECK-NEXT: [[REM:%.*]] = srem i8 [[X]], [[Y]]
-; CHECK-NEXT: [[R:%.*]] = and i8 [[REM]], 8
-; CHECK-NEXT: ret i8 [[R]]
+; CHECK-NEXT: ret i8 8
;
%x = or i8 %xx, -2
%y = and i8 %yy, -4
diff --git a/llvm/test/CodeGen/ARM/select-imm.ll b/llvm/test/CodeGen/ARM/select-imm.ll
index 65288e1884c742..6427a3e34cf8e2 100644
--- a/llvm/test/CodeGen/ARM/select-imm.ll
+++ b/llvm/test/CodeGen/ARM/select-imm.ll
@@ -655,14 +655,11 @@ define i1 @t10() {
; V8MBASE-NEXT: .pad #8
; V8MBASE-NEXT: sub sp, #8
; V8MBASE-NEXT: movs r0, #7
-; V8MBASE-NEXT: mvns r0, r0
-; V8MBASE-NEXT: str r0, [sp]
-; V8MBASE-NEXT: adds r1, r0, #5
-; V8MBASE-NEXT: str r1, [sp, #4]
-; V8MBASE-NEXT: sdiv r2, r1, r0
-; V8MBASE-NEXT: muls r2, r0, r2
-; V8MBASE-NEXT: subs r0, r1, r2
-; V8MBASE-NEXT: subs r1, r0, r1
+; V8MBASE-NEXT: mvns r1, r0
+; V8MBASE-NEXT: str r1, [sp]
+; V8MBASE-NEXT: adds r0, r1, #5
+; V8MBASE-NEXT: str r0, [sp, #4]
+; V8MBASE-NEXT: adds r1, #8
; V8MBASE-NEXT: rsbs r0, r1, #0
; V8MBASE-NEXT: adcs r0, r1
; V8MBASE-NEXT: add sp, #8
@@ -719,7 +716,7 @@ define i1 @t11() {
; ARMT2-NEXT: and r1, r1, r2
; ARMT2-NEXT: orr r0, r1, r0
; ARMT2-NEXT: str r0, [sp]
-; ARMT2-NEXT: bfc r0, #12, #20
+; ARMT2-NEXT: and r0, r0, #15
; ARMT2-NEXT: sub r0, r0, #3
; ARMT2-NEXT: clz r0, r0
; ARMT2-NEXT: lsr r0, r0, #5
@@ -781,7 +778,7 @@ define i1 @t11() {
; THUMB2-NEXT: ands r1, r2
; THUMB2-NEXT: orrs r0, r1
; THUMB2-NEXT: str r0, [sp]
-; THUMB2-NEXT: bfc r0, #12, #20
+; THUMB2-NEXT: and r0, r0, #15
; THUMB2-NEXT: subs r0, #3
; THUMB2-NEXT: clz r0, r0
; THUMB2-NEXT: lsrs r0, r0, #5
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