[llvm] b4c9146 - [AVR] Fix miscompilation of zext + add

[Ayke van Laethem via llvm-commits]

Author: Ayke van Laethem
Date: 2020-06-18T16:51:37+02:00
New Revision: b4c91462e84e66684c09d36c3c4ee9a10b9cb2cb

URL: https://github.com/llvm/llvm-project/commit/b4c91462e84e66684c09d36c3c4ee9a10b9cb2cb
DIFF: https://github.com/llvm/llvm-project/commit/b4c91462e84e66684c09d36c3c4ee9a10b9cb2cb.diff

LOG: [AVR] Fix miscompilation of zext + add

Code like the following:

    define i32 @foo(i32 %a, i1 zeroext %b) addrspace(1) {
    entry:
      %conv = zext i1 %b to i32
      %add = add nsw i32 %conv, %a
      ret i32 %add
    }

Would compile to the following (incorrect) code:

    foo:
        mov     r18, r20
        clr     r19
        add     r22, r18
        adc     r23, r19
        sbci    r24, 0
        sbci    r25, 0
        ret

Those sbci instructions are clearly wrong, they should have been adc
instructions.

This commit improves codegen to use adc instead:

    foo:
        mov     r18, r20
        clr     r19
        ldi     r20, 0
        ldi     r21, 0
        add     r22, r18
        adc     r23, r19
        adc     r24, r20
        adc     r25, r21
        ret

This code is not optimal (it could be just 5 instructions instead of the
current 9) but at least it doesn't miscompile.

Differential Revision: https://reviews.llvm.org/D78439

Added: 
    

Modified: 
    llvm/lib/Target/AVR/AVRInstrInfo.td
    llvm/test/CodeGen/AVR/add.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Target/AVR/AVRInstrInfo.td b/llvm/lib/Target/AVR/AVRInstrInfo.td
index b66eb792a2b7..f03c254382b4 100644
--- a/llvm/lib/Target/AVR/AVRInstrInfo.td
+++ b/llvm/lib/Target/AVR/AVRInstrInfo.td
@@ -2048,8 +2048,6 @@ def : Pat<(add i16:$src1, imm:$src2),
           (SUBIWRdK i16:$src1, (imm16_neg_XFORM imm:$src2))>;
 def : Pat<(addc i16:$src1, imm:$src2),
           (SUBIWRdK i16:$src1, (imm16_neg_XFORM imm:$src2))>;
-def : Pat<(adde i16:$src1, imm:$src2),
-          (SBCIWRdK i16:$src1, (imm16_neg_XFORM imm:$src2))>;
 
 def : Pat<(add i8:$src1, imm:$src2),
           (SUBIRdK i8:$src1, (imm8_neg_XFORM imm:$src2))>;

diff  --git a/llvm/test/CodeGen/AVR/add.ll b/llvm/test/CodeGen/AVR/add.ll
index cd3a69e6454c..30f23b199371 100644
--- a/llvm/test/CodeGen/AVR/add.ll
+++ b/llvm/test/CodeGen/AVR/add.ll
@@ -45,6 +45,17 @@ define i16 @add16_reg_imm_subi(i16 %a) {
     ret i16 %result
 }
 
+define i16 @add16_reg_reg_zext(i16 %a, i1 zeroext %b) {
+; CHECK-LABEL: add16_reg_reg_zext:
+; CHECK: mov r18, r22
+; CHECK: clr r19
+; CHECK: add r24, r18
+; CHECK: adc r25, r19
+    %zext = zext i1 %b to i16
+    %result = add i16 %a, %zext
+    ret i16 %result
+}
+
 define i32 @add32_reg_reg(i32 %a, i32 %b) {
 ; CHECK-LABEL: add32_reg_reg:
 ; CHECK: add r22, r18
@@ -65,6 +76,21 @@ define i32 @add32_reg_imm(i32 %a) {
     ret i32 %result
 }
 
+define i32 @add32_reg_reg_zext(i32 %a, i1 zeroext %b) {
+; CHECK-LABEL: add32_reg_reg_zext:
+; CHECK: mov r18, r20
+; CHECK: clr r19
+; CHECK: ldi r20, 0
+; CHECK: ldi r21, 0
+; CHECK: add r22, r18
+; CHECK: adc r23, r19
+; CHECK: adc r24, r20
+; CHECK: adc r25, r21
+    %zext = zext i1 %b to i32
+    %result = add i32 %a, %zext
+    ret i32 %result
+}
+
 define i64 @add64_reg_reg(i64 %a, i64 %b) {
 ; CHECK-LABEL: add64_reg_reg:
 ; CHECK: add r18, r10
@@ -91,3 +117,22 @@ define i64 @add64_reg_imm(i64 %a) {
     %result = add i64 %a, 5
     ret i64 %result
 }
+
+define i64 @add64_reg_reg_zext(i64 %a, i1 zeroext %b) {
+; CHECK-LABEL: add64_reg_reg_zext:
+; CHECK: mov r30, r16
+; CHECK: clr r31
+; CHECK: ldi r26, 0
+; CHECK: ldi r27, 0
+; CHECK: add r18, r30
+; CHECK: adc r19, r31
+; CHECK: adc r20, r26
+; CHECK: adc r21, r27
+; CHECK: adc r22, r26
+; CHECK: adc r23, r27
+; CHECK: adc r24, r26
+; CHECK: adc r25, r27
+    %zext = zext i1 %b to i64
+    %result = add i64 %a, %zext
+    ret i64 %result
+}